Well, McLovin pretty much nailed it, but, I'll ad this....
I've owned 2 6 series a 3.3 and 3.5, both had lsd and were 5spd. Both needed suspension and trans bushings to keep the ride crisp. One I hot rodded and put a turbo on
I love these cars, I also has a 535is that rocked.
They need a little ocassional love and attention similar to our 911s. But they are worth it in my opinion. Love the handling of the 6 series and the added performance of the 3.5 in addition to the 5spd. The autos are dogs compared to the manual.
I had to do a head gasket on one of them, at 250k miles!
Built like a german truck if there was such a thing.
Easy on parts, there are lots of recyclers and discount part outlets like Pelican and most importantly; some nice performance mods you can do that really wake them up. Standard stuff like intake and exhaust and a chip go a long way with bang for buck.
Be very thorough in the ppi process! Rust and neglect can bite pretty deep into the wallet. Don't buy someone elses problem, that 'deal' won't be worth it IMO.
These things are built like tanks! Fenders are glued then bolted on, beefy feel of the unibody too.
Buy carefully and enjoy it!
If the budget allows find a nice M6
Good luck!