I'd ask to see it running using water to make sure everything is working correctly. Take a look at the wiring and make sure everything looks good (e.g. no melted insulation). I wouldn't worry too much about the element(s)...they're really cheap, so don't be too concerned about corrosion or scale. Maybe focus more on how they're mounted and on how easy they are to replace. Pumps and controls would be my biggest concern. Make sure all buttons, switches, and lights are working.