My first was A Chevy C65.
It came with a rack dump, a very versatile unit! It was built with GM 366 tall block motor. Had problems with holding timing. On the advice of a mechanic, I decided to rebuild the engine. My engine rebuilder felt it was a shame to rebuild a 366. He contributed a 427 block to rebuild. It was 5 speed Eaton with 2 speed Eaton rear. Uphill, or down it was lower gear all the way. It drank gasoline!
I now have a fire truck low mileage. I took the radiator out for recore, had a mishap when a hood cable released unexpected, so the grill is out for repair at the moment.
You probably need CDL to drive it. If you will tow a trailer over 10,000 LB rated you certainly will. The air brake question is a tough one. If I interpret correctly, in my state, under 26000 LBS no air brake endorsement needed.