We have had both a Class C and Class A's. After experiencing both, here are my observations. If you are going to buy used a couple of suggestions.
1. Pay particular attention to the condition of the roof, ceiling and walls. Water intrusion is the number one issue of older units and if previous owners, like many, did not have annual maintenance performed, i.e. resealing seams, there will be water damage. look for any stains on the ceiling inside very carefully, especially in closets and slides. Water intrusion in the walls shows up as delamination. Check for any bulges or soft spots, particularly under windows.
2. Consider a Class A over a Class C. Class A's generally drive better than Class C's which will make your travels far more enjoyable. The excessive sway and wandering of a Class C will quickly exhaust most drivers. This is due to the Chassis design. Class A's are a MH designed chassis, Class C's generally are a van of light truck designed chassis. Its been my experience that Class C's ride and handle like a truck which IMHO is not a good thing. Aside from the ride, most Class C's come on a smaller chassis sporting 16" rims and Light Truck tires. Generally the tires must be filled to the max pressure to handle the weigh of the MH. Many of the larger units are so close to the max weight, that its very easy to overload them and as a result suffer tire issues and failures. Class A's generally come on a larger chassis with 22.5" rims and MH tires, have adequate cargo capacity and more manageable tire pressures.
The list of differences between the two classes is endless and I could go on and on, and most not in favour of the Class C.
But these two points are at the top of my list,