I have gone through this selection process myself, and used to think that a folding stock was the cat's meow for a truck gun. After trying one out, I ended up realizing that isn't really the case. Tirod is right that folding stocks are overrated. If a folding-stock rifle would fit, a standard stock probably would too. Plus, most folding stocks are not very comfortable and do not allow for very good, if any, cheek weld. As was mentioned, they take extra time to deploy, which wouldn't be ideal if you needed to quickly reach behind the seat and put it into action.
I would just go with whatever standard rifle you prefer. Cheap is good, because of the theft issue, and reliable is good as well. Personally I have a field grade M1 that rides in the trunk sometimes, and a WASR 10/63 that does as well.
Personally I don't like short-barreled .308s. If you want something for that close of range, just go with a carbine caliber. It will be lighter weight, less recoil, and just as ballistically effective at that kind of range. 5.56 and 7.62x39 are both fine at 200m, even out of carbine length barrels. .30 carbine is just weak, period. Handgun rounds are even weaker. I'd say either an AR or AK with an aimpoint (so you can leave it always on) would be great.