I thought the mk12 is a semi auto sniper rifle to begin with, so your ideal setup isn't likely to be found (bolt action) so an AEG is what you'll have to stick with if you are set on the mk12. Your best bet is to get an M4 or M16 and just get the smaller mag and a longer barrel. Good luck for under $400 though (with the upgrades, G&G M4 runs for about $400+). Just gotta look in the classifieds and hopefully find good deal.
Amphibious Recon Specialist
Level 2 Certified Sniper