I have the next one down. The 3 series based convertble Alpina. 2.5litre straight six, rear wheel drive, limited slip diff out off the M3, 89-90 manual, lowered and stiffened.
My advice. Try getting a cheaper model first. Although generally any Beemer is a good Beemer is has to be in bloody good nick to pay $9000 for it. I paid £1250 for mine, with a new soft top roof for it. I have seen normal 5-series, of that age, for under a grand with reasonable miles on the clock. For your money there you should be getting a pretty low mileage, mint condition motor.
But they really are lovely cars. And now with the accendancy of Audi as being the Chav mobile of choice, Beemers are now cool to drive again. So say's Jeramy Clarkson. And he is god.
Edit -And fuel consumption is all about how you drive it. A manual will be easier on the wallet, you have more control over the engine. But even with an auto, if your light on the right foot, you'll be light on the fuel. And beleive me, you can tell the difference between a hard drive and a gentle one. Especially with these size engines.