I looked into this idea for a while as well, but didn't really find that it seemed very worth while. If it's hot outside it will be hard for an atv to move at slow enough of a pace for an extended period of time. Even if it has a radiator with an electric fan.
My solution was to buy an old tractor mower like the ones they sell at Lowe's. It's an MTD 46" with an 18 horse motor. Runs like a champ, and didn't look that bad after I gave it a flat black paint job. I spent a little more than $300. Just my two cents. If you do go that route let me know what you thought.