Look for a shorty with dual rear wheels if you want to pull a trailer. Check and see what axle it has. It seems most short buses are on a Chevy chassis so there is the possibility of a corporate 14 bolt full floater, a Dana 70, or a Dana 80 rear end. The Dana 80 would be best while a 14bff or Dana 70 are on par with each other. Any of those axles should be ok though. If it's a Ford you might want to make sure it doesn't have a Dana 61. I would far rather have a 10.25 "Sterling" rear axle.
You don't mention how big the trailer is you want to pull, but in either case plan on spending the money for a good Class IV or V hitch and a weight distribution setup. Even if your tongue weight is within the rating of the hitch it will handle much better with the weight distro setup.
The transmission is the last area I'd be really cautious. It wasn't uncommon to find a 700R4 under a GM chassis or an AOD under a Ford. I wouldn't tow anything heavy with either. You should be able to find a Chevy chassis with a TH400 which will be plenty stout though it will suck down the gas. If you find a new enough one (at least 1991, maybe needs to be newer) there is a possibility that it will have a 4L80E which is every bit as stout as the TH-400, but adds an overdrive gear which will help mileage a lot.
More details on what exactly you're towing would help.