The first gens Q45s are the one to buy. Don't buy the later gens from '96-'2000. They are slower and more blander.
The first gens <and I think second gens> were underratted from the factory <Infinti rated them at 290hp, but from what I have read, they make more than 300hp>. They are extremely fast and handle damn well for a car of that size.
However they have a lot of techno-goodies which will probably go wrong. They were the first ones to have a fully active suspension <expensive to replace/repair>. The engine is pretty rock solid though. Try to get a warranty if you buy one.
On that note, the older and rare M30s <yes M not J> had a sensor on the front of the car that would monitor the road ahead and change the suspension dampers accordingly. I think it was more of a gimmick than anythign.