Spoiler!
Killing Floor might be the game for you.
It's cheap, runs on an old engine so you don't need a monster PC to play it, and it's fun as hell. Once a mod for UT2004, Killing Floor is set in the UK and puts you and 5 of your friends against the shambling masses not only to survive, but to eliminate. Use weapons ranging from a 9mm pistol to a LAW rocket launcher to send the failed experiments of the military into oblivion. Choose one of 6 classes to rank up, or work on them all. With each new level you get new perks like enhanced damage with a specific weapon, or increased ability with healing or welding doors shut, or carrying more equipment, etc. This allows you to play at higher difficulties and defeat bosses easier. Since it's on the old UE2.5 engine there should also be plenty of custom mods and maps and updates coming along to keep gameplay fresh.
If you end up getting it, I'd be glad to play with you. My Steam ID is PAKSoldat.
