The 4.4 engine is a very solid performer. There have been very few if any issues with long term maintenance and durability, and the performance is very very good. The only downside to going with a 2001 versus a 2004 or newer is BMW introduced the valvetronic technology in 2004, which gave the engine both an improvement in horsepower and fuel mileage.
I can't comment much on the 3.0d, as BMW doesn't offer the diesel version here in the U.S.
