Kimberly, A 34 year old trailer is going to have issues.
What is important to understand about fibreglass trailers is that they are almost all repairable. I have only ever found three that I would not consider fixing, and I look all the time. Like Raz said, the floor is the most difficult fix. Look in the bins under the seats, both the dinette and the gaucho, (front couch / bunk bed). On a 4500, that is where the big problems can be found. If the wood in the bottom of the bins is good, everything else is minor in comparison.
Fibreglass problems are fairly easy to fix. The only problem is painting
can be expensive, especially if you paint
the whole trailer. You said it had a problem on the outside, (road side?) front left front corner. If that is on the lower half, I would just fix it, then cover it, and the rest of the lower half of the front of the trailer in colour matched truck bed liner. You will never see it again. Fixing fibreglass is a very forgiving process. Got a dent, add fibreglass. Got too much fibreglass, grind or sand it down. Repeat as necessary.
It really saves money if you do the work your self. Of course some things, like welding, should probably be done by a pro.
There are four major jobs that will eventually need to be done. Some may already be done.
The belly band:
These leak eventually. Especially in Canada. It will have to be removed, the metal clips ground out and the fibreglass repaired.
Water damage around the windows causes the wood to rot. It is almost certain that the wood around the windows will have some rot. Especially on the wet coast. Even if they don't leak, condensation can cause rot. The curtain rods are screwed into the plywood window frames on the inside. So are the curtain hooks, half way down the windows. Give the curtain rod brackets a wiggle. Some will feel like they are in solid. Some may feel like they are not in anything. That is where the rot is. This fix is not too bad. The windows come out after the screws are removed. The the wood is removed. The hard part of this is getting the insulation (Ensolite) off the plywood. If the wood is rotten enough, this is not a problem. Cut new plywood, clean the insulation and fibreglass, glue in the plywood, screw the windows back in, using butyl tape to seal. Done.
They eventually get loose and need to be reinstalled.
The frame, axle
This is just a maintenance item, like the suspension on your car.
The 4500 is a great trailer, I have two. The check list of things I look for is:
The front window rock guard. Is there one? What kind of shape is it in? These are hard to find.
The insulation. Is it separating from the fibreglass? Are there rips in it?
The door latch. Does it work? Does it lock, (key?).
. Does it work?
. Does it work?
The power converter. Does it work?
Are there shelves above the windows, front and back?
How do the cushions look?
Does it have a vent fan?
How old are the tires
Does the axle
How old is the battery
Look at the frame. surface rust is not a problem, but look for places that it may rust through. In California it is probably fine.
The belly band. Does it leak?
None of these are deal breakers, just help to establish what I would pay. On the low end, a trailer in poor shape $1500. On the other end, a trailer that needs nothing and has the four major items done. Maybe up to $10000.