Re: better to carry or tow extra weight.
What kind of bus? How much do you have in it already?
The way I figured it, if a bus was designed to handle 70 passengers, it had to handle 70 150lb high-schoolers to school and still be within normal weight range. That means it was designed to handle 10,000lbs of rugrats. Yes, it is an over-simplification (and probably off by a few thousand), but the gist is that you can put a lot in there before you need to start worrying.
For towing, I think that your biggest restriction is going to be your hitch. I had a hitch specially made to hold 20,000lbs and...well....I tried to flex that one time. I towed a 7,000lb RV multiple times and it felt like it wasnt even back there.