Welcome to the club! If you are really handy with tools, you should be able to replace the roof by yourself -- there are lots of youtube videos describing the process. The trick, of course, is in the joints between the walls and the roof. Eternabond and Dicor will be your new best friends.
Alternatively, I would bet that you got a really good deal on this trailer -- you might want to spend some of the money you saved by hiring a professional to replace the roof. That's what I would do, even though I am moderately skilled with tools and sealants. This is a job that has to be done right -- otherwise, more water will leak in and further degrade the studs and rafters.
Do you have an indoor place to work on the trailer over the winter?
Good luck, and keep us posted on your progress!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)