Harmony
The harmony hub at this point is essential for control of audiovisual equipment unless you want to only use chromecast. That may change in the future, but for right now harmony is just a really good low-cost solution.
Voice Options
I personally have found the Amazon echo more useful than Google home, although I have tried both. I’d recommend that you do the same And just return the one that isn’t as useful for you. Best Buy carries both.
Right now, there are a lot more integrations available with Amazon, The $49 Dot is a great way to add additional units around the home, grouping options are much more precise, and The Google Home only has two microphones and it’s designed to be placed against the wall. The echo and Dot have nine microphones all the way around the unit, which let you place it anywhere in the room and it will be able to hear you from all sides. In fact, you can use only Dot if you want to, and just connect a speaker for any place where you’re going to use it to listen to music, news, and audiobooks. The dot’s own speaker is just fine for home automation commands, but it’s a little tinny for longer pieces.
I’m not saying don’t get the Google home but I am saying try both and see which one you like best. 
Home Automation Control
At this point, although it’s disappointing to say so, I don’t recommend smartthings to anyone who really needs the home automation to be reliable. Since November 2015 I have yet to go nine days without the system requiring some kind of maintenance, and since I have to pay other people to do that, even just popping the batteries on a device, I just can’t afford It. Plus I need automation that works.
So personally, I have shifted all of my critical use cases to apple’s HomeKit. I have Lutron Caseta light switches, a Philips Hue bridge and some smart bulbs as well as their motion sensors, a couple of Elgato Eve sensors, some ConnectSense pocket sockets, and a smart lock.
The Lutron Caseta and the Phillips hue also work very well with echo so I actually do most of the voice control through Alexa, but I use HomeKit for scheduling and sensor triggers. It has many fewer features than a SmartThings based system, but it’s reliable and requires almost no maintenance. I have had to do firmware updates on the lock twice in 15 months, but honestly that’s been it. Everything else is as close to “set and forget” as I’ve seen.
Most of the people that I know who require accessibility features use iOS devices anyway, and HomeKit comes free on any of the newer iPhones and iPads, so that didn’t add extra cost for me.
If for whatever reasons you don’t want to use iOS devices, there are still other systems that are more reliable than SmartThings at this point. The company is very aware of the issue and have said that they want to improve in this area, but they just aren’t there yet.
It’s true that a lot of the maintenance is just simple fiddly stuff, like rebooting a device by taking the batteries out and putting them back in, or opening the app, changing the time by one minute and saving it again to get a rule working again. So a lot of people hardly even notice this requirement. But in my situation, I notice it a lot. So I ended up bumping reliability up higher on my requirements list than I did initially.
Seeing who’s at the door
If you have the ability to physically rotate your wrist about 20°, enough to trigger an Apple Watch, and you could afford an Apple Watch, then HomeKit will very soon offer the ability to get the Camera notifications on your wrist, which from my point of view is by far the most convenient. I have a tablet mounted on my wheelchair, so I could use pretty much any video doorbell or camera at the door, it’s just that it takes four or five interactions to open the camera view and see who’s actually there. It’s OK, I just want more. 
So I’m expecting that to be available within the next few months based on multiple company announcements.
I’m really looking forward to that one. But I don’t know yet whether you’ll be able to clear it by voice or whether you’ll have to touch the watch face with your nose or a knuckle. It just varies by app right now. So that’s useful for some people, not for everybody. Otherwise the tablet apps are fine, they just take longer.
Emergency Notifications/Communication
Don’t use SmartThings for this. At all. It’s just not reliable enough. 
I have three separate systems set up, so I have redundancy.
First, again, is the Apple Watch. Really like this. I can do handsfree voice phone calls or voice text, again just by tilting my wrist. I don’t have to touch the watch but I do have to be able to Roll my arm enough to wake up the watch. This does not require lifting your arm up off the armrest. Just rolling it forward, although you don’t have to talk as loud if you can get your head closer to the watch when you’re speaking. I use this all the time, way more than I thought I would.
Second, if you have a mobile phone on the AT&T system, there’s an Alexa skill you can use to tell the echo to send a text for you. It’s OK, not perfect. But it’s a good example of one of those integrations that right now echo Has and google Home doesn’t. You can preprogram a fixed message and do that through IFTTT, so you can do it that way with either echo or Google home, but obviously it’s not as flexible.
AskMyBuddy uses preprogrammed messages, rather than actually sending a voice text. It’s a free service, which is nice, so it’s definitely worth trying, but if you do happen to have AT&T you get a lot more flexibility with their Alexa skill.
askMyBuddy:
Your Name Sent you an alert via AskMyBuddy.net, on Amazon Echo. This alert is sent only if this person has asked for help. Please check on them now. Once again, please check on Your Name, now! Thank you
The Att skill isn’t perfect, but it’s nice to be able to send an actual message, not something pre-formatted.
https://www.amazon.com/AT-T-Services-Inc-Message/dp/B01N3NPMHL
Third, I do use a panic button communicator Where if the button gets pressed I get two way voice communication with someone at the medical monitoring center who has a file with all my information in it. I’ve modified this slightly so that my service dog can push the button as well. This uses cellular monitoring, so it works whether I’m at home or away. It does have a $20 a month subscription fee, but I consider that well worth it.
Anyway, there are a lot of options for emergency notification systems, just again leave SmartThings out of the mix for now until the reliability improves.
Automatic Door (physically move the door, not just unlock)
I’ve looked at these, but don’t currently have any, because my service dog opens doors for me. There aren’t any that integrate directly with smartthings anyway, and I probably wouldn’t want to do it that way because of the reliability concerns. These need to have a lot of safety features so that they don’t smash you.
Drive medical has a unit at about $300 which is basically like a roomba with a slot tray that attaches to the door so it can pull the door or push it. It’s very very slow, but works OK for interior doors. There’s basically no installation required, you just Hang it on the door so the wheels touch the floor. It has an optional $30 one button remote.
For Exterior doors there are a lot of options, but one of the most popular low-cost brands is Skylink Otodor. They have a lot of different models. You can set that up with a motion sensor on the interior, but obviously you don’t want to do that for the exterior. They sell pushbutton remotes. These have more features than the drive medical unit and can work on more kinds of floors, but installation is considerably more complicated.
If you can’t do the push button for either of these, there is a “robot finger” microbot from Naran that you can add for $49, plus you need their $89 bridge (one Bridge can handle up to 40 micro bots) to give you IFTTT access and then you’re ready for all kinds of different integrations. Including echo or Google home.
So again, there are a lot of different ways to do this, and a lot of local companies that can install automatic doors for people using wheelchairs, so it mostly just comes down to budget. The more you can spend, the more elegant the solution will be.
Buttons
I have the ability to manipulate my power wheelchair to bump into a soft button on the wall. As it happens my knee height and my dog’s nose height are about the same. So we do enjoy using some of these buttons in addition to the voice option. Not everybody will be able to do that, and some people might want to put them down at toe height instead and bump them with your foot. It just depends on your own physicality and how you fit in the chair. But I did want to just mention that as yet another option. The Logitech POP buttons are big soft buttons that work well for this.
I’m not suggesting you get one as part of the starter set up, but if you find right away that you really wish you had a button, not just voice, it’s well worth considering. But not cheap. The starter pack with one bridge and two buttons is usually just under $100.
https://www.amazon.com/Logitech-Starter-One-Touch-Control-Devices/dp/B01JO8TIH4
But these are soft rubbery big buttons, almost half the size of a smart phone, so they work really well for a knee or elbow push, or a dog’s nose. I just wanted to mention them because a lot of the other switches are hard plastic and just not suitable for this kind of purpose. Voice is great, but sometimes you want a quiet option that isn’t a motion sensor. 
Where to Start
At this point I recommend you start with the harmony home hub, the Phillips hue starter kit with the plain white bulbs (it comes with 2 bulbs), one Philips Hue motion sensor, and your choice of either a dot or Google home.
https://www.amazon.com/Philips-White-Starter-light-bridge/dp/B014H2OZAC
Both Best Buy and Amazon sometimes have a bundle price on the harmony plus one dot which is worth considering.
https://www.amazon.com/Logitech-Harmony-Hub-Echo-Generation/dp/B01M9E5EB5/
Just those pieces will be enough to start trying out voice controlled home automation and seeing whether it’s something that might work well for you.
If your voice slurs at all (mine does) make sure you do the voice training that’s available through the Alexa app. It makes A big difference in the ability of the device to understand you.
If you happen to have an iOS phone or tablet, you can also try HomeKit with the Phillips hue bridge. Like I said, I use both HomeKit and Alexa.
So that’s where I would start. Just the harmony is great, but having the lights and motion sensor as well give you more of a sense of the possibilities.
The harmony is a little tedious to get set up. It’s obvious what to do, they just take you through a lot of screens to get all your channels set up and everything. The physical device itself is easy, you just plug it in to power, it communicates wirelessly with both your Wi-Fi network and your AV devices. But the app is just tiring to get set up. It will be worth it, though! 
The Phillips hue app is better for both the lights and the motion sensor.
Setting up the echo/dot is a breeze.
So just with those pieces you’ll have enough to play around with for a couple of weeks and get a sense of whether it’s going to be practical for you or not. Then you can start planning bigger projects!
well, those are just my first thoughts. As always, different things work for different people, and you might end up with a very different selection of devices than I would. And my main helper is my service dog, who can’t swap out the batteries in a sensor. If you have 24/7 human aides, you may be willing to put up with more unreliability in order to get fancier features. Choice is good.
Let me know if there’s anything specific I can answer.