I would get a carb rebuild kit first, before replacing it, you could also look at the classifieds on Craigslist or Facebook I know there are guys in my area that buy broken ATv's and strip them to sell the parts. You might be able to find a good carb, and the front differential for cheap.
One of my firends growing up had a '87 big bear 350 and it was a good machine, you really can't go wrong with an older Honda or Yamaha. I'm looking to pick up a '95 big bear 350 with a plow for around $1300, I would say you got a great deal if the plastics aren't broke apart.
Can't help with the manuals, sorry.