Just to pipe into the conversation - my house is just under 200sqm and 2 levels and while 1AP did cover the whole house (mounted centrally downstairs) it didn't give good 5Ghz coverage in the corner bedrooms. I've got 2 APs now and that's perfect - one downstairs on the North end and one upstairs on the South end. If you're running ethernet backhaul then you'll probably want them as far apart as possible like that.

I'm running my controller on a Mac Mini I have setup to run Plex and some other things which works great. You can just use the controller for setup and updates and then not have it running all the time but then I do think that you miss out on what's so great about Unifi in doing that. I'd much rather have it running all the time if possible.

If all you want is a couple of good Wifi devices for each corner of the house then the Spark Smart Modem + Smart Mesh is actually a really good choice and will cost much less. 2x equivalent Unifi APs will cost you $700 all up where as Spark will give you their modem for free and the Mesh unit for about $100ish I think. Once it's setup properly it's about as good as the Unifi APs, just not as configurable. Just thought i'd mention as you said he's not super nerdy about his networking.

And yes to your thoughts on setup. You'll need the ONT and Modem in the garage and then ethernet in the APs in whatever room makes sense to have them in. If you have power sockets next to where the APs will be you can use POE Injectors or do what I did and have a POE Switch in the cabinet with the ONT and Modem and then you don't need the injectors. Just much tidier, but more $$ of course. 

Be careful which APs you buy too as some come with injectors and some (like the In Walls) do not. They're cheap enough to buy though.