It sounds like this thing is in pretty decent shape, at least on the outside.
As for the price, I would recommend that you check New Cars, Used Cars, Blue Book Values & Car Prices - Kelley Blue Book
to check the reasonable private party purchase price for this vehicle. The price will depend on a number of options.
As for the towing capacity, and perhaps another member could chime in here, I would have to estimate that the capacity for that stock bumper (if it is the type which accepts the trailer ball) would probably be between 2,500 and 3,000 lbs. This is pretty standard for a base 1/4 ton rated truck. Your tent trailer should be well below that limit and you should have no problem.
I would always recommend a trany cooler for any tow vehicle, especially one such as this which would be totaled if your trans went out. on that note, it is probably the most expensive part to repair on that vehicle. I would suggest at the least, checking the condition of the trans fluid (should be bright red if its an automatic) and checking for any abnormal shifts while driving it.
As for common problems, there is one which is most obvious on these vehicles: cracked cylinder heads. From what I understand, the 2.9L heads were quite prone to cracking, probably when an engine overheat occured. While this can be difficult to see, I would suggest taking this thing to a shop before you buy and have them perform a compression test. A cracked head will leak pressure (and power), and possibly coolant.
Not sure if anyone out there has any good broncoII links, but a bit of searching might sort it out.
Bottom line: Barring any unforseen problems, a truck that old which looks good, probably is. I might try to talk him below $3,000, but I bet he was hoping to walk away with 3K in cash anyways. Do your research, having the bluebook printout is a powerfull tool.