I never believe salesman, they will say anything to get you inside and then try to sell you something else. Who knows someone might have put a deposit on that car a long time ago and when you arrive the salesman says, "Oh so sorry, but let me show you...." classic bait and switch.
If no codes are being thrown use Chevron Techron fuel injector cleaner (concentrate). It solves rpm fluctuating upon cold start-up. Also, for most BMW problems start off by scanning your car with the Peake Research Tool. It contains the actual BMW codes. If you want to register a newly installed battery for free (just buy a $10 cable) and google/download BMWLogger