Welcome! Sounds like an exciting project!. 
There are two very different ways to go about planning a smart home. And separately, there are two very ways to think about budgeting for it.
Hobbyist or Problem Solver?
Some people come to smart homes because they are fascinated by the technology. They read about all the different devices and often tend to be early adopters, sometimes even contributing On Kickstarter to models which aren’t even in the prototype stage yet.
They are often fine with a system which needs a lot of tinkering or customising, while at the same time they tend to have An attraction to a “ One ring to rule them all“ approach and can get quite annoyed if they have to use multiple apps.
They also have a fervent belief that if a device can do something they ought to be able to make it do that, and do that in integration with all of their other devices. They will spend a lot of extra time, and even extra money, in order to unlock these “advanced“ features.“
At the same time, they can be surprisingly tolerant of a system that has multiple glitches as long as it is versatile and powerful overall.
The second group are people who have a specific problem that needs to be solved. Whether it’s a simple as giving a dog walker access to the house in the afternoon but not wanting them to have access at other times, or as complicated as a household with a family member with Alzheimer’s who wanders, They know exactly what problem they need to solve, they just aren’t sure how to do it.
They generally don’t want to tinker with the system after it’s set up, although there are exceptions to that. And reliability is really important to them, because the solution really matters to them.
And because they don’t want to tinker with it after it’s set up, they often don’t care if it takes multiple apps to get the problem solved. They are focused on the solution, not the process. If multiple hubs means better reliability once the solution is in play, they’re usually fine with that.
In some ways that boils down to whether your personal priority is versatility or reliability, but there is a bit more to it than that.
If you saw an amazing new product at an introductory price, would you buy it first and then try to figure out how you were going to use it? if so, I would put you into the hobbyist category. Problem solvers have a problem they are looking for a solution to, and then select the devices on that basis.
Of course, someone could be a bit of both, it does happen, but you can see why these are two different strategies when it comes to automating a home.
Budgets
There are also two different strategies for budgeting home automation projects, and while there is some correlation to the hobbyist/problem solver categorisation , there’s more to it than that.
The first group sees each device purchased as an investment, a “sunk cost.“ It’s not about how much they paid: some will only buy budget devices and others will buy premium devices. rather, it’s about whether they felt it was good value and in particular their expectation of how long they expect that device to perform After installation.
Some people buy smart light switches and motion sensors as though they were light fittings and plumbing pipes. They somehow expect the devices will work for 10 or 15 years with about the same life as their dumb counterparts. While it’s true that the warranty on most of the smart devices will only be one or two years, they still tend to get infuriated if a smart switch stops working after four. They also generally expect that their hub will work forever, even though the terms of use on almost all home automation smart hubs say that features can be added or removed at any time.
For whatever reason, I think this group is the majority of home automation adopters, whether they are hobbyists or problem solvers. This group spends a lot of time Worrying about futureproofing their investment.
The second group sees Home Automation as a service and the devices as the delivery mechanism for that service And expect to replace individual devices on about the same schedule as they replace their mobile phones. And are prepared to replace the hub every three or four years as well.
They set a monthly budget, again much like they have for mobile phones including device replacement cost, and then go on from there. As long as they don’t go over budget, they are OK with changing out devices when new ones come along with new features that would work better for them.
They don’t change just for the sake of change, but they are prepared to throw out a smart switch or sensor or even hub if there’s something better on the market and it’s in their budget to do so.
My story
I am a network engineer, and psychologically tend to lean towards the hobbyist side, but in recent years I acquired a neuromuscular condition and I’m now quadriparetic. I use a wheelchair and have a limited hand function. That means I have to pay someone else to do almost any tinkering, including just changing the batteries in a device.
So I have, by necessity, moved to the problem solver group in my approach to home automation. After my first year, it became apparent that I had to put reliability at the top of my requirements list, and that changed a lot of things for me.
At the same time, the technology is changing rapidly and costs are coming down on a lot of advanced features overtime. So I budget on the home automation as a service strategy.
I have a certain amount I put aside each month for Home Automation and I expect any device I buy, including a hub, to have delivered full value after three years. If I get more use out of it, that’s great: it just means more money in the budget to try new things. But I don’t try to futureproof anything, even door locks, beyond three years.
That saves me a lot of stress and, in my case, saves me money in not having to pay other people to keep old patchwork systems running.
If you’re curious, I budget for Home Automation about the same as I do for a mobile phone and service, around £100 a month. That has to cover any ongoing subscription fees, The devices, and the hub as well. That would seem like a lot to some people and not very much to others, but it works well for me.
I started out using the other budgeting method, planning to spend up to £365 Per room and a total for the whole house of around £3650. ($500 per room and $5000 for the house for those in the US.)
But I pretty quickly realized that My numbers were right for the initial investment, but I had to accept that I might have to replace a high percentage every three or four years. So over 20 years my real budget was going to be much higher, but it could be broken down into that monthly cost of around £100 ($125).
And I thought, OK, if that solves The problems I need solved, I can manage that.
So today I am a problem solver working with a monthly Home Automation as a service budget.
If you’d like to see the details of the big problems that I needed solved, I posted a project report about that:
Adding Home Automation in Phases: my limited investment strategy
Someone else might have a budget of £25 pounds a month or a budget of £200 a month, and they might base it on a two year replacement cycle or four years or even five. The point isn’t the specific numbers. It’s whether you are comfortable with the idea that you might have to replace anything or everything in a relatively short period of time. as people do with mobile phones. Or whether your budget expectation is more in line with rewiring the ceiling fittings: barring a major disaster or a major renovation, you see it as a one time sunk cost.
Well, that’s enough to start, but I would begin by thinking about whether you are going to be a hobbyist or a problem solver in the way you select devices. And how you want to approach budgeting. once you know the answers to those questions, we can start talking about specifics. 