FWIW, I'll never have a pickup bed again. The storage and utility of a flatbed is just too good to be true.
Utility body versus flatbed? I would ONLY go the utility body route if I knew exactly what I wanted to take with me and especially if I had things that needed to cut through the "plane" of the deck suck as a spare tire mounted vertically or large tanks of some sort (propane, water, natural gas?). If you aren't 100% sure, go with a flatbed. And just to be contrary to BKA, the guy that setup out rig for his South American journey, spent one month getting it ready. So it is possible.
If you want the flexibility of removing the upper boxes, they can be done that way though your idea of plastics totes etc is reasonable. My upper boxes are removable but it is a bit of a chore so I don't do it, but I could relatively easily.
My one recommendation on the flatbed itself, is to get one with stake pockets so you could mount a 2x4 (or x6/x8) as a removable side rail to help hold your boxes in or other gear when you are TC less.
Also in the FWIW department, most flatbed a are 9' long but in at least our rigs' case, the last 10" or so could be chopped off and allow the TC up against the headache rack.
Did I say I'll never have a pickup bed again?
If I was doing it myself, I would likely find a flatbed used. They are around and you'll end up welding to it, maybe cutting it, adding tie downs points or other stuff so just starting with a used one saves you a lot of money and makes you worry less about modding it.
Good luck!
(Got rained out on our Highline Trail hike (Glacier National Park) today popped in here to check in. Hope everyone is enjoying their TCs.....you sure do see a lot up here!