I'll suggest two. First one is a bushmaster m-17s. Bullpup configuration, uses standard m-16 mags. Modifications that need to be done to it, IMHO, are adding a picatinny rail on the under side for a foward grip and drilling out vent holes in the aluminim receiver surrounding the barrel. The gun gets to hot to handle after about 3 clips without prolonged cooling. Uses the gas system similar to an AK-47 so fouling is not an issue as it is with the M-16/M-4 clones. No longer in production, never a "popular" seller. I see them all the times at gun shows for 500 to 600 bucks.
The other is a FN FS2000. Also a bullpup. Also takes standard nato mags. Haven't owned one like I did the M-17, but I want one. Modular foregrip configuration (white light, laser, grenade launcher for military) so its slightly customizable, at a cost to your wallet. Runs about 2k new. Brass isn't ejected, its collected and pushed out the front. Charging handle can be mounted right or left. All controlls are ambidextrous.
If you want something in .308 (7.62x51 nato) I'd say go with a CETME or a FNFAL. Tons of aftermarket parts to make it your own. Reliable track record on both.

-Bill Liptak