You might want to check out the FAQ regarding green gas, all you really need is a can of propane from wherever and an adaptor for the propane outlet.
Green Gas = Propane
There are a lot of M14's in the classifieds and they can range anywhere from a couple hundred to $800+ (including a few upgrades, mags, etc.).
If you're a history major and just want it as a showpiece all you should be looking for is a cheap M14.
I would suggest however that an M4 might be a better starter weapon as you can play CQB and outdoors. Whereas the M14 is really more of an outside weapon because of it's length.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE