Get a special order X5 35D and get a bigger trailer for more tires, fuel, etc for the GT3.
I am sure that BMW will allow you to spend lots of extra money for the only X5 35D built the way you want. Full nappa, every option, individual package, etc
That would be my choice, but I am not you.
Ah, but really...by the X5M from the start. Don't get the 5.0, if you are going to get a fast one, then get THE fast one.