Well, if you EVER want to play with AA, don't get the KAR-98. It shoots well over our FPS limits.
Seeing as how you are only 15, I doubt you can even afford a VFC M82A1. But even if you could, I have heard bad things about it.
Of the three, you'd be better off with the M700 AICS.
Now, I personally don't have much experience with gas sniper rifles...but I had the chance to mess around with a gas L96A1 that belongs to Brian(IcePlatinumSky) and it is SWEET. Accurate, consistant, incredible range and sweet looking, while remaining completely stock.