Polaris small block 600s have been the one bright spot in their lineup since they started putting them in the Edge model. The engine has no bad manners and gives you pretty good HP for trail riding.
The sled should be plenty capable for the weight, the 2-ups have extra springs to accommodate two riders. With only 800 miles, you have a LOT of miles left in it!!
2-ups don't come up for sale very frequently around here, and when they do, they sell pretty quickly. The mid-NADA price sounds fair, you can offer less and see if the guy takes it. Never hurts to ask