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Welcome!
You don't "need" anything but rather the question is what do you want to change/improve upon and what is your budget?
The M Performance Brakes Upgrade gives a significant improvement with much better brake feel. Even with no data to back it up my impression is that the X5 stops better and gives extra confidence compared to the stock brakes.
Suspension wise there isn't much you could do besides lower it with ACS/Eibach/H&R springs which is mostly for looks. I would be shocked if springs did much for improving handling honestly.
You can pretty much just put 21-22" wheels on the X5 and not do anything else really.
Alan
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