M cars can be expensive propositions out-of-warranty. I would have a certified or dealer mechanic look over the car and then base my decision and offer on their advice. I think you want to be particularly careful to check whether any modifications to the car affected the warranty. If you decide to pull the trigger, since the car is well under 50K miles, you also may want to consider purchasing an extended warranty (or at least a maintenance warranty). As for the SMG, I like it and have had ZERO problems. There seems to be a bias against this system, but it strikes me that that's all about personal preference. There's nothing wrong or catastrophic occuring with the SMG.
First and foremost, have the car looked at by a BMW mechanic!
2011 Panamera 4S...GT Silver / Espresso
2013 Porsche Boxster S; 2012 Mini CooperS
convertible; '04 GX470
Prior BMWs: `05 645CiC; `07 650i; `08 M6 convertible; '08 135i; `11 750ix MSport