Financially why buy one?
M6's to lease are cheaper than M5's, M4's are significantly more than both. The pricing is all over the shop.
You'd be better sticking 45k in a bond for 5 years and leasing a brand new M5 and handing the keys back in 2 or 3 years.
Depreciation, insurance and servicing will be catastrophic on one of these second hand when the lease deals dry up. There will be a glut of M5's on the market...
Lease one, spec it exactly as you want have BMW pay the outgoings and pay 13k over two years and get 2k from your principle.
Just my 2p. Good luck whatever you get!
* and use 2k for your lease deposit