Three types:
M91/30 Long barrel (very long)
M38 Short carbine (I have this model)
M44 Short carbine with folding bayonet.
Caliber 7.62mm x 54 R (the R stands for either Rimmed or Russian I think)
You should be able to find a decent one for less than $100. For a truck gun I would recommend the M38 or maybe the 44 just because they're shorter.
The power is on par with a 30-06 and they are plenty accurate.
ETA: When inspecting a possible purchase, use a bore scope, or light. Most of these have never seen anything but corrosive ammo. Make sure the bore isn't rotted out. Also, learn to clean the thing after firing corrosive ammo. Basically rinse the bore with windex immediately after use, then clean as usual.