)
As already mentioned, model/year/engine will impact maintenance costs. If you are relatively handy, normal maintenance isn't bad, just don't expect to get most model's filters or brake pads from Autozone, Advanced, Napa, etc. I buy most items off the net and turn the wrench myself.
As for the top not working, could be something as simple as a bad motor or sensors, again, if handy with repairs, might just be a few hundred $ for parts and a day of your time. I had a bad top hyd motor on my Z4, was quoted $1300, did it myself for $350 ( the Z4 requires removal of the entire top to gain access to the motor in the bottom of the left wheel-well) - stupid design as it sits right beside the roof drain and most fail due to water damage
BMW has a history of extra gadgets and sensors, even with early models, so that can add a little time and headache to basic maintenance, but still quite manageable.
The upside to all of this is, when maintained, they run like a top!
Oh, and I get 26 in town and 32 highway with a 3L inline 6
Shoot, you'll be fine then; like Hayata mentioned, the BMW forums are a wealth of knowledge - if someone has had a problem, there is almost guaranteed to be a DIY post with pics to get you through anything.
A K&N high flow air filter (believe it or not) helps a tad (about 2-3 over stock)......... but I also swapped out the intake manifold from my stock 3.0i for the 3.0si manifold; luckilly, its one of the very few engine models that only used the intake manifold and different ECU programming to get the added HP
