You already have the car so theres not much that you can change now.
Maintenance can be not too bad if you get lucky and don't have any problems. FInd a local shop that works on BMW's and get everything done there. If a turbo or two god forbid, goes its going to cost an arm and a leg no matter where you get it done. Just make sure to save enough for unexpected repairs.
No one is going to have an accurate answer for how much its going to cost. Totally depends on what goes wrong.
Regular maintenance shouldn't be too bad though.