building a sniper-grade upper, etc.

[iomatic]

Forgive the newbishness. Looking for recommendations, thoughts, etc. Trying to stay cheap, naturally. Also would like to build a .22 upper at some point for ultra cheapness.

Since I have a complete, I was thinking I'd just swap out an upper to do sniper matches. Is that an acceptable approach?

-20" barrel (no idea who, specs, or whatnot)
-free float rail (lightest? or...?)
-obviously, optics; probably the most expense here?

-trigger job on the lower (would probably suit all types of shooting)
-or-
-cheap lower to feed the upper build

Or do people just figure, spend the $200 on a complete cheap lower, and eat that cost?

Thanks!