As above, just to chuck my 2p in - the 320d is quite frankly crap compared to any 6 cylinder model. It sort of made sense when the car was relatively new and you "just want a BMW with reasonable economy" but now they're getting on it has next to no merits (for me)
The 330d/cd is a much better engine, but you take an economy hit to the point where it's not all that much better than the petrol, or at least not really enough to offset the potential for bigger bills (by all means you could own one for a couple of years and nothing go bang, but if it did it'd be expensive). If you were going to take a chance on an e46 diesel it'd have to be one of these imo.
The big 6 petrols are refined, pretty economical and sound better than the diseasels. There's also less of a premium as everyone else pretty much wants a diesel car.
You can't really argue about 35+mpg on a run from a na petrol car with 230bhp imo.