I know it sounds like we're saying "get all of it!" But really, get the sub, too. The sub is one of the key difference makers in how it sounds. I can turn off my sub on my system (which is a good system) and it totally lessens it.
Get the front three in a matching timber (compatible), plus a...
The one set of speakers that he has.
I'd get a good center, use the mains for a while until he can afford new ones that go well with the center.
Actually, I'd get all three in the first buy.
Where's the center speaker? I'd think of it as the single most important speaker, figure out where it's going, what kind it is. Then get a good L and R to match it.
For your surrounds, bookshelf Polks should work -- mount one on the wall to your left, even with the couch. Could do on a...