Going in to A/C shop in the morning.

Car started making a metal grinding sound about a month and a half ago and I just parked it and haven't drove it since. Got a few of you on here who said my symptoms were a/c comp. Guy at shop gave me a quote of $700 for comp. dryer, etc. Just wanna know is there anything I should do like ask for the old comp. or anything just to make sure im not getting beat? Just incase its NOT the comp and he changes something that costs 20 bucks and calls it $700??? SURE its the comp though cuz i got same symptoms as a few others. Grinding sound, smoke under hood, electrical sort of smell... Any help as always would be greatly appreciated.