If you're going to be brush painting, get yourself some Vallejo Panzer Aces colors. They brush fantastically well, and the Panzer Aces line has pretty good color accuracy. Though I'd highly recommend priming before painting (rattlecan auto primer should do the trick). You can thin Vallejo with their thinner, with Future (Pledge w/ Future Shine), Testors acrylic thinner, even Windex or windshield fluid - but the ammonia in those two does attack the paint versus thinning it, so use sparingly (and definitely use it for cleanup!). One thing with Vallejo - it's one of those acrylics where you don't want to use any kind of alcohol as it'll cause it to clump.
As far as building and painting, that's definitely a judgement call. My personal habit is...anything that's the main hull color, and can easily be painted while attached, gets attached. Bits that need a different color (the rubber on many tank road wheels, stowage, machine guns) and bits that will be difficult to cover (fuel drums on T-34s as one example) get painted separately and attached later. But everyone has their own preferred way of doing things.
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com