Hi Melinda and welcome to the RVUSA Forum. This question has been asked numerous times. You can search the forum and find more answers later, but I'll try to address yours here.
1. Dealer or Private: If you want (some) security, buy from a dealer. A dealer has a brick and mortar address and a reputation to maintain, so if something goes wrong with the RV (and it will) you can take it back to him for repair. You will probably get a better deal money-wise from a private seller. However, if something goes wrong, you're on the hook for the repairs.
2. Prior usage doesn't mean too much, as long as the RV has been properly maintained. Some basic rules of thumb are 5 to 7 years on tires, no matter how good they look; 3 to 4 years on the house batteries and 7 to 10 years on the roof, depending on what type it is and how well it has been maintained. Check the interior really good for evidence of water leaks and test every appliance and system to make sure they all work.
3. Boss Lady and I have a 31 foot Class C and it is adequate for the two of us. Just remember, after 3 or 4 days of rain, even the largest RV can become confining,
Hope this helps some. Enjoy your search for your new (to you) RV and post back if you have more questions.