It'll need new gears, the piston will shit the bed later on in life, prepare for the trigger to be crunchy, sometimes mushy, don't count on being able to get any decent standard mags, don't get bothered when it tears your cutoff lever up kicking down from full auto to semi, et cetera.
I know some about M14s in airsoft and I really like the TM one I have. It's the nicest gun I've ever owned.