As far as weight...
anyone with a horse (who really cares about them) will make sure they can tow at least two, plus tac and gear and the weight of the trailer. Then they'll usually increase that weight - just to make sure nothing fails while hauling their animal around. At least thats how it is down here.

The point being... it'll probably haul 10k lbs on a flat road comfertably.
That 3 speed is going to kill you in fuel though.
to give you *SOME* semblence of price...
I recently drove an 85? 4 Door W350/360/3sp auto 2100 miles with about 600lbs of cab cargo (humans, blankets, pets) and about 400lbs bed cargo. No trailer...
We averaged about 12mpg. 1 tank would get us about 200 miles, give or take 20 miles. That's with a 360, so a 318 would have been a little better on gas, but not a lot. Average might have gone up to about 14, MAYBE 15.

If you havn't bought the thing yet, look for a 93 318/5sp auto D150... it'll pull the weight. You might want to install a Reese hitch instead of using the bumper, and make sure the trailer has electric breaks so you're not having to replace the trucks every turn around. You'll get a LOT better gas mileage this way. BTW, I happen to have one for sale... needs a bit of body work, maybe a new paint job... $3k though and it's yours

 

Sounds like you have a bumper mounted ball and not a real hitch?

Pics definately.

 

A 318 and a 411 rear end?? DON'T look for fuel economy....LOLOL

 

if you put the right perfomance parts in the engine you can get pretty decent fuel economy even if you are pulling a trailer.