Some more details about the repairs that need to be done with pictures would help. I'd start with running gear so you can tow safely: tires
, bearings, and lights
. Make sure to check dates on the tires
. Trailer tires
usually age out (6-7 years) before they wear out.
But for now cleaning up the trailer is a good start. Bar Keepers Friend, a very mild scouring agent, works well to clean up the fiberglass. Buy the liquid if you can find it, but if not you can turn the powder into a thin paste with water.
Once the exterior is thoroughly clean, you can go several routes to restore some degree of gloss. Cheapest is regular automotive paste wax. It will bring back a dull shine, not a high gloss, but it will protect the fiberglass. The first time you may want to do several coats, and buffing out will be difficult because the surface is rough and somewhat porous. After that if you keep up it gets easier. I wax twice a year, spring and fall
A step up is marine wax from 3M or Maguires. Better shine and longer-lasting, but still won't be a high gloss finish like new gelcoat.
For that, many folks use a liquid floor wax. I forgot the specific product names, but I'm sure others will chime in. It give a true high gloss and can last 1-3 seasons depending on climate. But it has to be stripped when you redo it.