V7... The gearbox itself is good for beginners, its straightforward and simple. Sourcing parts is not, as the gear set is a proprietary ratio. Pistons and cylinders are also proprietary. The M14 itself is annoying to take apart.
V2, V3 and V6 would be good for beginners to learn on.
The Lancer Tactical is V2 compatible, you can learn on it without fear of wasting something valuable. It will be annoying due to the poor parts fitment. Working on shitty guns is a lot more time consuming than working on a good gun.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN