used "C": how old is too old?

The older they are, the more important issue is water leakage. We bought a 10 year old Minnie Winnie. We wouldn't trade it for any of the new ones. Newer ones have been designed to cut corners or less quality construction.
We found a water leak in the overhead which almost everyone has especially those that have a window in front.
Refrigerators, and heaters go out but no one can say when. Both still work in our now 14 year old C.
If you do a search just for 'older class c' here, you will get much more info.
Over all, I don't think anyone wished they had done otherwise.

Hope not due to my investment of triple of your budget for one that was 13 yrs old. Bottom line I wouldn't consider more than 20 years old and very dependent on overall condition. I've seen all types of campers from Class A to popups and some were only 2 to 5 years old and completely shot and others over 20 years old that were in show room condition.
Regarding the appliances, they are built to last a long time, as long as the owners have performed proper service and used them correctly. (ie keeping the RV level in order for the refrigerator to perform properly)otherwise yes 100000. for the right C is ok, but what size what condition, what make/brand has a lot to do with value.
before committing, you should post info about the unit your considering and see what the feed back is.

Buy old and cheap but in good shape. Your first one will probably not be your last one. You will learn about floor plans, appliances and driving and what works for you and what doesn't. It shouldn't be an expensive lesson. If you pay $10k for it now, it will be worth close to that next year if you decide you want something else. Or if you like it, you've saved a lot of money.

Eric