I was in a very similar situation to Aaron about a year ago... Like old things, like tractors, so interested in buying an old tractor. Simple! I too was interested in a diesel, but like others said before, most tractors at the price/size are gasoline. I kept my eyes open and ended up with an AC B (1942). I agree with Cop, buy restored (or close to it). Mine was not recently restored tip-top, but it was gone over well not very long ago. Good enough to not give me any headaches yet! It came with an old rotary mower, so I've given it some work in addition to it's main purpose, joyrides with the kids. The seat is quite wide, so it works great for this!
I've used it to take wagon rides for 6-8 people without concern. I don't know how much weight everyone is thinking for a typical "hay ride" but I believe my usage has stayed within the abilities of the tractor. True, the brakes are not very effective, but the property is mostly flat and the tractor is heavy enough with the wheel weights for an enjoyable stroll.