Welcome! Home Automation is a lot of fun, but definitely it can also get overwhelming. For some of us, like myself, it is also a necessity (I am a wheelchair user and quadriparetic), so that’s yet a different angle.
People approach it in many different ways, but two basic viewpoints.
-
technologists. These are people who see a really cool new device, check out the features, fall in love with it, and then say “… I wonder what I could do with this?” 
for inspiration, they will check out project reports from other people, blogs, the manufacturer site, looking for things that maybe they never would’ve thought of doing on their own.
-
problem solvers. These are people who have a specific “use case” that they want to solve. Maybe they want the lights to come on as they pull into the driveway. Maybe they want to let the dog walker into the house without giving him a key. Maybe, as you said, they have some thing that they want to have happen when the temperature changes. So they will first think about the problem they want to solve, and then look for the right combinations of devices that will solve it.
Of course both groups overlap, sometimes you’ll have a core set of problems you want to solve and then you start getting into just fun browsing of new devices. And both groups can run into budget or practicality issues as well. Or of course regional issues, but since you said you had GE switches, I’m assuming that you are in North America. If not, let us know, because the device selections do vary.
So…
As far as your specific question, your home automation system has to know what the temperature is if it’s going to act on a change in temperature. Your switches don’t know by themselves and neither does the hub. So, yes, you will have to get some sensors if you want to have temperature-based routines.
As far as not getting overwhelmed, just take it one step at a time. You don’t have to do everything all at once. Think about how you’d like to focus your research: do you want to see the latest and greatest new devices? Or do you want to look at how others have solved specific problems?
There are project reports listed in the community-created wiki. There’s an “impress your friends “list, with everything from a secret room to a magic wand. And there are practical lists, organized both by room (kitchen, bedroom, yard, etc.) and by function (lighting, HVAC, etc.). There’s also a “getting started“ list with topics like “the 10 best things to do with smartthings“ etc. so lots of places to get ideas if you’re interested.
Just be aware that smartthings is right in the middle of a major transition from one architecture to another so some of the specifics will be done in a different way than they were if the project report is more than two or three years old. But if it could be done five years ago with smartthings, it probably can still be done now, and you can always come back to the forum and ask about current specifics.
FAQ: Using the quick browse lists without the community wiki
Anyway, have fun, research as much or as little as you like. And if you’d like to give us some more specifics about exactly what you’d like to do with having a fan come on, we can just kick that idea around. Will need to know if you mean a plug-in fan or a ceiling fan, if you have or want a smart thermostat, what kind of budget you were thinking of, and then we can start to get more specific.
You might also talk to your friend some more and see what exactly about their home automation his family ended up not liking. There are a lot of reasons why something like that will happen, the most common being unreliability, but it can help focus your own ideas as well.
p.s. because I use voice to create my posts, you will see some weird random capitalizations, there may be some odd word substitutions (“corset” instead of “core set,” for example 
) , and it may take me two or three edits to get everything set, including links and images added. So just please be patient with me and understand that while my initial post may appear very quickly, it might be a few minutes until the final version is complete.