I bought a Sportsman 400 (425 cc's actually). I tested it side by side with the 500, which is identical except the larger motor. I did'nt really see $800 difference between the two. A 350-400 class machine would do everything you would likely want it to. Definitely get a liquid cooled machine for any type of work like spraying, mowing, towing, etc.
Having said that, there is no such thing as too much power. I have since bought an ATV disc and while the 400 pulls it ok, extra HP would just make it easier on the machine. You generally run out of traction before you run out of power but with discing or similar chores I would take all the HP you can get. I know you already have a tractor though, lucky dog!
A 4wd ATV is an extremely handy tool for anybody who owns more than 3 or 4 acres. I plow snow with mine, spray, mow, landscape, foodplots, check trail cams, retrieve deer...