Don't buy on mileage as such, buy more on condition and the transparency of the car. By this I mean that you can be certain of it's history with only one or two careful owners. So for instance, rather than going for a 50k mile tired looking GTI with a few owners I'd go for say a 65k mile pristine GTI. Of course the difference shouldn't be too big. Mileage won't kill the car, lack of care and age will!