I bought my ATV with something like 1000 km on it, so if you are patient, you will be able to find the right deal.
If you do buy something with high mileage and you want to do some work on the engine, there is a great course offered by Algonquin College (Woodroffe Campus)
http://www.algonquincollege.com/ccol...small-engines/. I took the course a few years ago. It is offered at night, I think once or twice a week, and it runs for a few months. During the course, you bring in a small engine and work on it under the supervision of the instructor and some technicians, using their shop tools. You do simple things like rebuild the carburettor, but you also do more complex tasks like change the head gasket, change the rings (if you want), check bearings, polish the cylinder etc. Depending on how fast you are, there may also be time at the end to work on other parts of the ATV. The advantages of taking the course is that you are forced to set aside some time every week to work on the machine, and you have the instructor's expertise available for all the repairs you want to do.