I don't think that there is a "best" for exhaust. I personally like the THS Racing exhaust. It has a good sound and adds some noticable punch. I would check Ebay for the f-650 body. Hobbieguyrc or rcboyz probably have one. I just ordered my ext f-350 body from hobbieguyrc.. i think it was $19 or so..
I bought the sirio tuned pipe from my LHS for $40. Very noticable difference. Pops wheelies now. Particularly it doesn't come off of the header, it actually slides onto it about a half an inch, Therefore it's not going anywhere.