Ok if you are doing that mileage I would go for a lower mileage car as you will soon rack all the miles up. If you go for the 60,000 car then in two years time you will get to 90k and you might have to pay for things to be put right.
Just to give you an idea I've got heated leather seats, panoramic sunroof etc (the only thing I don't have is sat nav) and when I got the car it had done 11,000 miles and cost me just under £12. The higher the spec the more it's going to cost so maybe look around the 40,000 mile mark taking into account your budget?
Believe me you won't be disappointed with the cooper s. It's great fun to drive. You'll just have to have patience to find the right one!
Hope that helps