Thanks everyone for the input.

Here's what I think I'm hearing:
- I should check the service records and the GT1
- Because it's the last model year of the e53, I should be better off (and the fact that it's had one owner doesn't hurt)
- The older the car gets, the more likely there are to be problems (obvious, I know)
- Ultimately, electrical problems on x5's are unpredictable and unavoidable

A couple more questions:
- Is it worth asking to see the CPO worksheet?
- Does having extra "electrical" options increase likelihood of issues? (nav, parking distance control, cold weather package, etc.)
- Are there any possible warning signs for future window issues?
- Is it worth considering a third party extended warranty?