Diesel? I thought the bmw 325i touring was a petrol?
First up I'd say improve the brakes, Z3 brakes should fit ok and you can get these from breakers yards nowadays.
Personally I'd drop in a panel filter if I were you. Induction kits with a cold air feed can make about 8bhp on these but I quite like the standard engine noise rather than the roar of an induction kit.
I know of a few guys that have dropped in the cams from the US spec M series into their 325's and these give a reasonable power gain. Another option is to replace the pistons and crank from the 328.
When you've done your mods get a remap done and this will fully realise the cars potential