if your 540 is 1997 and is 6spd than a Mark D tune will make the most difference for the money. If you car is 1998+ than a tune doesn't really do much. 1997 540's were made in the M-factory and use larger intake manifolds from factory, later on 2000+ DINAN E39's used 1997 intake manifolds.
1990 M5 Euro
You wash your car like it was your firstborn child, you tend to its needs like it was your own body, you protect it like it's your family, then you drive it like you stole it