The 530d is without a doubt your best choice. The diesel engine in it is one of the best in the world, and Evo Magazine actually thinks it is only beat out by the M3's I6 when it comes to non-supercar engines. It is also ligheter weight than the V8, and has massive tuning potential, propane works like Nitrous on diesels and is cheap. K&N filter will gie a couple ponies, but chipping one of those bad boys can bring some massive gains. Turbo charging is expensive, but pays off big. The stock car is no slow-poke itself though, just listen to some numbers:
And as we all know, torque is more important than HP when it comes to acceleration, heck HP only determines top speed! This thing is fast off the line, and just pulls like no other. And don't just take my word for it, test drive one.
If you can't find a 530d(They are rare), then a 535/540i would be your next best bet. Price wise, it depends widely. I would suggest trying to get a certified used 5 series, they still have a waranty, and you can trust the car is in sound mechanical and aesthetic shape. Be cautious of the first year of the e39 though, some bugs were still being worked out, and '99 was a bad year for BMW's aesthetic reliability rating(re-tooling).
Good luck, and welcome to the BMW family in advance!