You need to check your door tag because i bet your over 10k gvw.... that means your going to have to use a log book. Good luck getting signed with a expedit company because not many are taking on vans/box trucks.
On this forum there are tons of posts on van builds even photos. You will be better off building your bed and sleeping area. I would start out with a simple set up and make sure you get a espar heater and a good ac with min 12k btu and good generator thats both small and quiet(3k watts)like a...
The rear end is the easiest change to do and reprogram the computer.....if you change the transmission you need the computer for it AND drive shaft then reprogram it....you can even change tire size and get different fuel mileage.
You still want to weight the van with you, your gear and a full tank of fuel. Take the total and subtract from your door sticker. That will be your true weight limit.
A transit 150 might be to light.
You +your gear+full tank take that van to a scale at a truck stop and weight it subtract that weight from the driver side door sticker....thats the weight your van can legally carry.