I purchased a TT that is over 20 years old. It's built solid, was well cared for, and everything works. Bought it in the fall for a little less than $4,000 from a young family that wanted to upgrade. I inspected inside for water damage. Using a bright LED light I checked everywhere including opening all the cabinets and closets. FYI - there are several Youtube videos that show how to inspect an RV for water problems.
I crawled under the TT as far as I could checking for signs of water leaks and rust. I checked the tires which were all fairly new. Talked with the owner who was forthright in answering my questions including that the house battery was a cheap non-deep cycle battery. And, another very important item was that the I liked the floor plan; it would work well for me. Also, before inspecting, I did an Internet search to get the cargo capacity and GVWR to make sure my TV could tow it and it had sufficient cargo capacity for all my camping stuff.
Since it is an older TT, I assumed I'd need to spend a few hundred dollars doing updates and installing not necessary things to make it comfortable for me. However, if I wanted, I could have taken it out camping immediately after the purchase. Turns out I didn't need to spend much on needed updates which included replacing the converter/charger, buying a study hydraulic jack in case of a flat tire, 2 deep cycle 6 volt batteries, and heat gun to check TT tires and brakes for excessive heat when traveling. It came with a good weight distribution hitch, but I had to buy an anti-sway device. Total of about $800.
So, my recommendation is to pay attention to the floor plan and whether you like it and it will work for you. As others have suggested, closely inspect for water damage. Take a ladder and couple of pieces of plywood to inspect the roof and check for uncaulked seams including around the vents, a/c, etc. Be sure you have enough cash after purchase to pay for updates and other things you'd like to have. Does it come with a weight distribution hitch with included sway control? If not, you'll need to pay about $500 for a WDH with sway or $50 to $100 for a friction sway bar. Other stuff will cost a whole lot less. For instance, I paid $190 for an updated converter/charger and about $200 for new house batteries.
Result is I have a nice TT that is fully paid.
Good luck finding a TT that will work well for you. And, my compliments for deciding not to go into debt to purchase a recreational item.