Ok here is an i dea I had for a GCS Vehicle Combat game with a Vehicle selection of sorts.

Theory
: Replace the punch with a miniscule or invisible md2. make the punch do no damage, and make no sound. Second relace weapons with different vehicles MD2s that would suit the desired weapon. Keep them all centered on the screen and a sort of top/back view of the vehicle, animate them propperly. Besure only the front portion of the vehicle is seen as not to disorient the player because of width. Third create the first level as a parking garage type thing with md2s of the weapons( not accual pickup items) that will teleport the player to the respective parking garage. Have the player step toward the vehicle and be prompted to press enter to take this vehicle, there for equipting the respective weapon. it would be a good I dea to have the spot where the vehicle is also have an animation to adjust the players speed accordingly to the vehicle chosen. This should remain connstant throughout the entire game. Leaveing the garage should trigger the first stage. Utilize the entire area that GCS provides. Keep details down to a minimum as not to spoil framerate. enemies only need one frame for movement , or more if ya wanna get detailed and a dieing animation. make ammo for each weapon available and "respawning" if possible
Downside
: Each "vehicle" will only have 1 weapon.