NAC quoted me $4K for an extended warranty that takes it up to 100K miles. It's their Pinnacle plan that would include almost everything - certainly the important parts / those prone to issues. This was substantially less than comparable plans offered by EasyCare and BMW. I'd recommend calling them (Google for contact details) to see what they can do for you - if you decide to keep the M5 it will also help with eventual resale.
The F10 M5 would also be nice. Don't get me wrong, the F10 550i is a great car - owning and drving it will "feel" very different to driving around in your M5, though.