for WWII guns, unless you want to convert a M14 to an M1 Garand, the following would be your best bet to buy. again, let me know what side you like to play on (Axis or Allies).
AGM MP-40: A solid little gun from the reviews i have read. Mags can be hard to find, as can specific replacement parts. I know that one guy on this forum was custom making steel cut-off levers or something like that.
AGM MP44: I have fired this a few times. those i know who own it love it.
here is a great review on it: http://lrrphk.proboards.com/index.cg...lay&thread=225
I think this would be your best bet, though some upgrading of internals and downgrading the spring would be necessary IMHO. still, its a V. 2, and that means MANY replacement parts. Its also a caged V.2, which IMHO is a big plus
Thompson: This is a good bet as a starter rifle. Made by TM, so you know its good. Its a little pricy, but worth it.
Ares PPSH: With this gun coming out, you can bet there will be a lot of spare parts available.
Hope that helps.
If you have any questions on specific guns ask away. Those are just my opinions on what to get as a starter rifle.