You can't really know what they are selling you. You can look for signs of work done. Is it a reputable shop? Is it a 660 or 700 originally? Is the price good?
The motors on these aren't built to last forever with the things we want or expect out of our Rhino. What kinda riding do you do? If your duning it all the time, your working even a stock motor hard. Treat it right you can get many seasons out of even a built motor. And with loads of fun along the way.