It really comes down to what you want to spend your time on. For example, I'd love to convert my Eurovan from a gasser to a TDI. It might make sense as VW never sold a TDI Eurovan here in the states. I'd have something unique and it would give better fuel mileage.
On the other hand, if I converted my Jetta TDI to a VR6, that would just seem silly. I can sell the TDI and buy a VR6 ready to go.
If you were going to convert your 330Xi to a diesel, I can see that. Unique vehicle not available in the states. Plus you can upgrade that diesel to make plenty of power. If I was going to do a BMW project, then it would be converting a 330Xi Wagon to a diesel.
So creating an extremely rare vehicle does has it's merits. A Steel Blue 330i is extremely rare. A Steel Blue 330i without ABS and DSC is probably a one of a kind. So, I'd be taking out full coverage on it. If someone steals it and takes it on a joy ride, they will certainly lock up the rear brakes (as a BMW without ABS has too much rear brake bias) and total it.