I have owned three 850's and i dont know why everyone is scared of them.
I would never buy an 840 - Nikasil issues and you don't get to watch people's faces when you tell them its a V12. (yes I know the 840 has nominally weight distribution).
If you are expecting the running costs to be in the 3 series range look elsewhere. If you are comparing it to a comparable v12 exotic it's a bargain.
The only really expensive part of ownership is the Inspection 2 , mainly because it requires changing the plugs, wires ($600 or so for each bank from the dealer), cap and rotor (one for each bank).
Gas is also a killer, It is easily the worst car I have ever owned for gas mileage. it will give you almost 30MPG when you are set on cruise control on the highway...down hill with the wind blowing favorably. City driving with any sort of enthusiasm will see you in 9-12mpg range.
The M70 engine is pretty much bullet proof. Just make sure that the car has the updated rad cap - or else there may be headgasket issues.
Auto Transmissions can start to let go around 160k (damn BMW lifetime fluid), but there are plenty of low mileage cars around that you dont have to waste your time with a high km car.
Like someone else mentioned do not try and buy one that hasn't lived a pampered life. Buying a neglected one can cost more than the inital purchase price to put right.
Don't buy an auto and try and swap to a 6speed unless you have acess to lots of cheap rare parts and lots of mechanical skill/know somebody who has lots of mechanical skill that will work for next to free. Much easier and cheaper to just buy a 6 speed.
The biggest problem with them is that i'm pretty sure they are cursed.
Everytime i get a new one something always comes up that makes me need to sell them.
This time it seems to be the itch to buy a 911 before I turn 30.