Well I dunno what to say. I have a 33 foot that I have pulled a 30 foot trailer that weighed 11,700 loaded. It is a Chevy chassis with a 454 gas motor. I just had to beef it up a little but not much, I did it with scraps from around the shop. It pulls my 28 with 3000 pound Nova, golf cart, scooter, big tool box, generator and all at 75 with one hand on the wheel smooth as silk.
I say the bigger the better but for a gas unit I wouldnt recommend over 32 or 33 feet. If diesel go nuts...lol... To me that looked like a fair price for a unit in that good of shape.
I couldnt imagine going to the track with one shorter than mine and especially not with a truck after having the MH...