Get either a SAR-1 or SAR-2 like everyone else says. Definately the most bang for your buck. The only difference between the two is caliber(7.62 and 5.45).
Each caliber has it's advanteges and dis-advantages, though I'm partial to the 7.62, since I only have an SAR-1.
7.62-Slightly cheaper ammo (about $20 less per 1k)
-Cheaper and more plentiful mags
-Drums
-More recoil (can be good or bad depending on preference)
-Drums
5.45-Flatter shooting and slightly more accurate
-Less recoil (I hear it's easier to bump)
-It's what the Russians upgraded to
Of course, AIM is always a good place to get one. That's the best price you will get for an SAR-1 or 2, and the SAR-2's are cheap now there.
IMHO, you can't go wrong with either caliber, which leaves you with one solution: buy both!