My advice is look for rust in the tailgate - pop the glass, look down at the right and left corners where water sits, a surprising amount of cars have it, even ones sitting on BMW dealer lots that I looked at. Other than that, I think you have the bases covered with your M5 experience.

 

Why isn't my M5 gone yet? New unicorn just showed up and it has nearly everything...even night vision.

2010 BMW 535 i xDrive, $24,241 - Cars.com

 

I saw that one. They have the car fax on the dealer website BTW. It sounds perfect. I'd love to pull trigger on it, but I have to haul two little ones around which means cream beige interior is a deal killer. My kids would ruin that interior in a week I'm afraid- little jerks. I'd have trouble keeping it clean over time even without kids.
Anybody having good results with long term wear on cream beige? Please say no- I've already convinced myself it won't work.
Good luck with your search. If I come across anything interesting, I'll post it here.

 

You'd have to do more work for sure, esp. on the carpets, but leather cleaner might do wonders... =)