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!
Currently: 2015 Porsche 911 GTS and 2016 Cayenne S eHybrid
Past BMWs: `05 645CiC; `07 650i; `08 M6 convertible; '08 135i; `11 750ix MSport; `15 M235i convertible
"You think that's air you're breathing now?"