CCA track days can help anyone to handle a car.
Maintenance & condition count for alot compared to mileage on these cars. Age dictates that both cars would require similar work to get them where you're comfortable with the car on the road. Check Bruno's bmwe34.net site for things that will break.
http://www.bmwe34.net/e34main/troubl...will_break.htm
Another reason to go for the cheaper 535 is the opportunity to spend quality time with the kid. The 17 year old needs a car that needs him. That $6k difference will buy a ton of maintenance.