Patryn,
The EXI plastic version of the 500 that Xheli sells is something I also have looked at. The plastic really isn't all that much.
For the head, you'll want to replace the mixing arms and washout arms (as they are plastic) - the rest of the head is metal alloy. I suggest getting CF frame sides to replace the aluminum frame sides because in a crash they will warp - I have read of guys having to remove them to straighten them out. That seems a bit...aggrevating!
The tail is all plastic so you'll have to read up on any issues with that. Also, the belt is associated with static buildup so you'll have to run a ground wire to the motor mount to take care of that. There is a large EXI thread for your reading pleasure [link=http://www.helifreak.com/showthread.php?t=156783&highlight=Xheli]Here.[/link]
Don't forget the usual stuff that may need replacing: shafts, main and auto gears, soft metal screws, etc.
Mark