I agree with wrgrs50s, get your friend to go over it all with you. Showing you how things work.. if you go to him asking for his help as you are a newbie, he should be more than willing to walk you through it all. If he is hesitant to show you things, I would take that as a warning that maybe something isn't right. I would not want to buy anything if the seller wouldn't demonstrate it.
Not sure about that brand, but some manufacturers have manuals and sale brochures with details about sizes, tanks, weights, and what came standard available as archive files on their website. Look for that and go ahead and start reading.
I sold a 10 yr old 24ft TT for $5000 a couple of years ago, so that is probably a fair deal. I wouldn't give him more $500 for the hitch, you can buy a new one for that. Maybe $100 - $150.