I picked up a rough but running 800 RMK for the track and skid five years ago. Well, even thought it ran, the crank runout was over .01. The clutch weight bushings were junk, and the weights had damaged the spider. But that was a mountain sled with almost 5000 miles. The old 800 "big block" is actually a pretty good, long-lasting motor if the clutch is properly balanced and set up right, and it hasn't been abused. Trail sleds tend to see less wide-open-throttle, so the likelihood of problems is lower. The left driveshaft bearing on that sled was almost destroyed, and I've had to replace that particular bearing on my 600 as well. I've replaced other bearings, but the two in the chaincase and the left jackshaft bearing seem to hold up pretty well.
Here's what I'd check and do: pull the carbs and check the carb boots (had those go bad, can cause poor running, and may result in a lean burn-down), and while you're at it, check what jets you've got and reset the idle mix and air bleed screws per the factory service manual (FSM). You'll probably need a clutch puller, although I rode for a couple seasons before I got one. You can inspect the primary without pulling it, but I don't think you can check the crank runout (that's more of a "should" than a "shall"). Check the oil pump and set it according to the FSM, then I like to drain the oil tank and refill it a pint at a time, making a mark each time. That way you can make sure you're getting the right oil mix by comparing gas burned to oil used. You'll need to bleed the line to the oil pump. At a minimum, grease all grease points on the front suspension and skid; it'd be a good idea to pull the skid, clean everything out, and pump fresh grease in; same on the spindles. For the driveshaft bearing, you can check it by taking the speedometer drive off. You have to pull the driveshaft to replace that bearing, but maybe it's been done. On the clutches, I'd disassemble the secondary, inspect it, and replace the spring, as well as the primary spring. Not sure if this is a PERC (electronic reverse) sled or not, but you may need to replace the secondary rollers or buttons. If it's been neglected, there's a weekend project worth of work, even for someone who's familiar, that probably should be done. For however much you're able to do, look for any weird noises and vibrations when you do start riding it. It may not need anything (if you're lucky), but even a little checking could save you a lot of trouble. Both my sleds were well cared-for, but were also relatively young sleds when I got them. Even if you're not ready to tackle everything we've mentioned, it should give you an idea of what to check. Definitely find a copy of the FSM, look things over, and don't take anything for granted. They EDGE chassis is kind of old-school now, but they're still great sleds and will go a long way if they're taken care of. Even one that's neglected can often be brought back into shape without too much time and money.