Following is the way I approach it and others may have different skills and be able to do some repair items that I won't. I evaluate a used molded fiberglass trailer by cost of what is needed. When those items are added to the purchase price and it exceeds the price of one in top condition then I keep looking.

The high dollar items are:

Axle replacement, appliances, foam and upholstery, tires and rims, structural and frame issues.

Shell condition(including windows) must be damage free and still be able to be polished back into good condition. Sub flooring is not a huge expense, but is high labor and I personally don't buy trailers that need sub floor work. Some folks don't mind sub floor repairs, just depends.

Electrical, plumbing, adding missing features, new vinyl flooring, painting or cleaning hide/rat fur, polishing shell, and needing top to bottom scrubbing are all items assumed to be needed and I don't worry about those because they're low dollar and almost all labor.

As you look, feel free to ask specific question on trailers you find, or any additional general questions you may have. Season ramps up in about 6 weeks and there will be quite a few for sale soon.