Another plus with the 4.6 is that it doesn't have the air suspension. If I were building one of these with a lot of power I would definitely want coilovers.

You could also look into engine swaps - the V10 would be a blast

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12 X5 M - 06 X5 4.8iS - 03 X5 4.4i - 03 M5 - 02 X5 4.6is - 99 M3 Dinan S3 - 98 M3 - 92 850i-6 - 92 850i - 91 325ix - 89 M3 2.5 - and a few parts cars