so I'm thinking about building an A4 style upper and need some advice if it is cheaper to build it myself or buy one. I have access to a KAK M5-ras for $50 with everything and that would be the rail but can barrels be found for $100-150 with the A2 post? Also what type of barrel should I get?

buy a 20inch a2/govt profile upper from PSA or Windham.

Would that be the bast bang for the buck? Trying to do $250 without BCG

You may find a used upper that is not chrome lined for $250 if your fortunate.

I doubt you will get a upper with chrome lined barrel for $250, even a used one.
This is about the best deal going right now for a A4 upper with a chrome lined barrel.http://palmettostatearmory.com/index.php/ar-15-05/barreled-upper-assemblies/psa-chf-20-5-56mm-1-7-rifle-length-a2-profile-w-bcg-charging-handle.html

Historically, even used chrome lined upper halves without BCG/charging handle go for $350 and up.