TM FAMAS isn't unreliable, it's just an old design that's not really supported anymore with aftermarket parts. By the time you hit 18, I wouldn't be surprised if someone has made an updated version.
If you're worried about length because of your height, I'd recommend a M14. Assuming you're not built like a stick bug, anyways. Though really, AK47, full size G36, full stock M16 and SIG550 are all potentially good choices for tall people.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."