You could do a 420G, but you'd lose the AWD.

I just checked and it looks like E53 got MS43 until the end of production so that's good if you wanted to S52 swap it.

You would not use S52 engine harness or DME to swap into the X5. You would use the M54 engine harness on the S52, and the MS43 DME. If you've done it before it's a weekend project. If you've never done it before it will take a bit longer.