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If I were you I'd start off with a Unifi Security Gateway and Unifi AP second hand. You should probably be able to get both for about £120. That's pretty much the same functionality as the dream machine but a fair bit cheaper and you have more flexibility on where you put your wi-fi point.
If you have a machine that's always on then you can run the controller there. I've run mine on Ubuntu, Windows and Raspberry Pi.
I haven't bothered with the switches (I had a toughswitch but gave up on it), they don't add that much for a home system I feel. Just the USG and AP allows you to set up multiple networks, block them from seeing each other, etc. You can just use an injector for the AP power (it will come with it normally).
One thing to be aware of is that different products come under different families and require different controllers which is really annoying. The Edge Router X (which is very similar to the Security Gateway) for instance uses a different controller to the access points and the two don't see each other at all in terms of managing them.
Where do I start with Ubiquiti to replace my ageing Nighthawk R7000? I like to tinker and not afraid of Linux.