Hi Tristan, and welcome to the club.
I don't think it is a strange choice for a 21 year old -- the XKR is a great car and a future classic.
I wanted a Jaguar when I was 20 but had to settle for a bicycle!. I still wanted a Jaguar [a Mark l and then a Mark ll] when I was 30, but had to settle for a motor cycle.
I finally got my Jaguar after a series of company cars, and giving it a good burst of power on the A38 is quite enjoyable and makes me feel as it I was still in my twenties!
Let us know how you feel when you drive it home.
Regards,
Peter.