yup its like F150, F250, F350 trucks but the E is for Econoline, Aka van.
350 seems to be the standard for older shuttle buses. 250 for some single rear wheel small models, and now with the newer models there is 450 and even 550 chassis. the higher the # the more beefier the chassis is. bigger brakes, stronger springs, more everything.
also 450, 550 is almost identical for towing/weight etc.
biggest thing is you want a good sized engine in it to. i have a 7.3L diesel in mine. yet the congo bus has a 460 gas engine in it but still lots of power for hauling.
What kinda bus are you looking at?