Well Danny Boy, I'm a fan of buying new and the biggest and best whenever possible. That's not to say I wouldn't buy used, but if I did buy used it would have to be something I was very very familiar with and knew the history of. There are times when $$$$$$ determine what we buy or don't and in that case we do the best we can. Around here used quads are usually very very used yet still command premium money making a used purchase not worth while.
Mileage is hardly realevant since which would you prefer, a quad that has 10,000 miles on it from cruising trails or 1 that has 500 miles from being reamed in the mud. Unless you know the history of the thing you're not sure which you're getting. By using my Prairie around home and just plain fun riding with friends I'll pile up 5,000 or so miles a year. That's as much as most people put on in 5 years that I'm aware of.
I guess my short answer is to buy as much quad as you can and as much as you need.