Eliminate the Time Capsule because it doesn't really function as NAS, not because you all use PC's. PC's can talk to them just fine. It's almost exclusively for backup purposes tho. Nice bit of hardware tho, we rock one at home.
For people with budget, I'd be looking at something like this:
http://ascent.co.nz/productspecification.aspx?ItemID=387740
But yeah, $800 bux.
I can't really advise on any cheaper options tho, as just about everyone I know who's gone through a few NAS has ended up with something from http://www.synology.com and they are about as expensive as the home server type option.