I agree with WhoKnownWho...if you have $600 to spend you should spread it out. You can get SAR-1s for $329 on Altantic Firearms, and a new Arcus 9mm for about $250 on AIM surplus. Makarovs go for about $150+, but I am more of a big pistol person myself.
If you want to go the shotgun route, there are some viable options that others can reccomend, though AIM surplus is selling UCN 1201 12 gauge pumps for $160. I'm no expert in shotguns, so I can't tell you much more.
An AR will cost you at least $650, if you invest in a flattop and a back up iron sight, which you definitely should. Trust me, you will thank yourself for getting that flattop instead of a fixed carry handle later when you go to put optics on it, or so I have been advised.