Echo the comments above re this sounding like a project car.

There was no 4.1, there was a 4.4. Changing the engine to a 3.0 would be complicated by the fact that they use different transmissions, and some coding may be required so that you don't get continual error lights on the dash. The software has to know what engine and options the vehicle has.

Washing it is a good idea, but skip the undercoating. All it does at this point is hold the salt in. Rust isn't an issue on these vehicles.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue