I was lucky enough to snag a custom built M4 off the classifieds.
If I were in your position I would just lurk the classifieds until an M4 that you like comes up.
Also I know that there are some players who sell on their local forums only (eg. JOC, and Op-For's B/S/T Section), check out your local forums and you may even be able to drive out and test it out before buying.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE