I'm not sure which transmission the '01 E450 uses, but on the 350s the later transmissions are a big improvement. Same for the V10 - the pre-03 motors tend to have the plugs that seize in the heads. IIRC, '06 and up have the extended reach plugs that just break in half. If the buyer can't prove that the plugs have been changed, prepare for the worst. They should have a 100K service interval. I think Ford gets around nine hours labor in the E series to change the plugs, they are that much of a nightmare to do.

Disclaimer - I tow a 28 enclosed with a '04 V10 E350.