The 645E is of relatively recent vintage, so it wouldn't be as bad as a 40+ year old camera that had sat unused for a long time.
Does it have the standard hand crank, or the thumb operated "rapid" winder, or both?
Check the camera and lens for signs of excessive dirt, visible migrating lubricant, excessive wear, mould or battery leakage/corrosion. If you don't see any of these problems, and the camera appears to function well with film in it, as long as you budget for a CLA, it should end up to be a good choice.
For the purposes of comparison, KEH is currently offering an EX graded 645E with lens and standard hand crank for $354: http://www.keh.com/camera/Mamiya-645...990628700?r=FE