If you're building a rear wheel for a fixie, you'll need a fixed-gear, or 'track' hub. You can also get a 'flip-flop' hub, which has a fixed cog on one side and can fit a single-speed freewheel on the other. You can switch between fixed-gear or freewheel by flipping the wheel around.
Don't know what kind of rim you're trying to use, but you can get a F/R wheelset from Pure Cycles for $100. 40mm 'deep-V' rims, too.
https://www.purecycles.com/collectio...ries/wheelsets
They'll sell you a hub, too, but WRT spoke length, we need to know your rim section depth, hub flange diameter, and intended lacing pattern.