I am kind of a person that will buy "junk" that I hope to make usable at least. if you are this type of person, let me state a couple things that goes against what the guys above me have said, just a little at least.
Any trailer, you buy used, probably needs tires and battery at a minimum, especially if you get one "cheap" where the seller is not fixing it up and trying to get top dollar...
SO battery and tires are a "wash" so to speak.
But water damage (and resulting black moldy) crap is a challeng. it probably leaked around skylite you described. you might get lucky and just started, but my bet is been leaking a while.
The ceiling wont be too hard, the problem is how much structural damage you might have with rotted wood frames or what not. (I am not authority on older, or any years hilo, but I used to build RV's and busses for Eldorado Motor Coach).
(I can see this one is going to be long, excuse my typo's, I have to hurry)
I bought a 72 Eldorado motorhome in 2003, it had leaked above the cab (overhead) could only see a small spot, I thought easy fix, it was "easy" but lots of labor involved though. I grew up farming, and told me working on something, is better than hanging out in bars, etc, or shopping, so any time fixing things up is double win win.
I had to remove 70% of the wood and frame above the cab, that is under that bed, and I cut and spliced in wood and frame back in, on any soft wood. I painted the hell out of everything with a "brand name that escapes me" paint that was a primer to kill any mold (think it was "KILLS"). I didnt have to go up the sides, the leak was a seam around front window on that over cab, so bottom of front window was as high as I had to go. We used that camper for the next 8 year, the camper cost me $1000, then I bought a few cordless tools I had always wanted anyway, and probably $200 worth of materials (very carefully sourced, like "dumpster wood" from friends, matress from garage sales, things like that, in wood, and paid a friend that redid my seat cusions for dining area, (if I had to pay retail that alone would been 200 or more? Plus, and my mother, who loves sewing and making shirts, curtains and lots of things, did the Curtains herself, all with material she'd VERY frugal person, had horded lol.
Only reason I sold that dang thing was the hard to get tires (16.5 rims), plus it was made in 72, so top speed felt like 50, and I was needing to travel 300 or more miles at least 4-5 times a year, if you know what I mean. And then I found the Hilo 5th wheel toybox I have now.
BTW, insurance on that RV was like $36 per year, and antique tags, makes it so I didnt have some $20,000 dollar camper (and payments... like my friends have) that I may or may not get to use each season.
So if you are able to do the work, Id offer less, take it home and work on it. But, if you need a camper for even a month from now, you arent as handy, or have nobody to help, you might need to pass, unless you have the time. it took me 2 weekends, full weekends and some week nights, to get that RV so I could use it right away...
Your mileage on this post will vary,