I would go for ANY of the six cylinder models.
I have an E91 320d (163bhp), this replaced an E46 330d (204bhp) I made the change to the smaller engine because of Co car tax. Big mistake, one which I constantly regret making.
It doesn't sound like a proper BMW, it doesn't have the grunt of the six cylinder engines and (in my opinion) marks the buyer (me) as a cheese pairing penny pincher.
The four cylinder diesel in a BMW is just wrong.
I now view 320d BMWs as the reps car of the year - a modern Sierra / Cavalier etc etc etc.
Buy the six cylinder engine - just check out the various video clips on this site (Carlos' one springs to mind) of the acceleration to 60 - it doesn't happen like that in the 320d - trust me I know.
You will regret it if you buy that 320d. I did and do.