Proposed Foamie Build

[Projector]

Imho, keeping the side rails is a plus for side impact and rub protection. Also solidifies the trailer and reduces flex.
I've thought about exactly what you've wanted to do and may still build a 6*12 cargo trailer. On the cheap RV living forums, they convert enclosed cargo trailers for those with less skill and time. Certainly quicker, faster and more durable.
I've just built a 7*14 foamie on a van cab and chassis and am not fully convinced that the time and cost have any benefits over a box van. While happy with the results, the durability doesn't compare to metal or fiberglass.
Ymmv.
As for the upper cabinets, go for it. I just did 8' long by 16" deep by 16" tall with only two sides added, using the existing wall and ceiling. Using 3/8" ply and 1*2 and 2*2" framing, light and strong. Your not going to be putting heavy items but store light weight things up there and the extra storage space will be very useful. Tie into the existing wall studs and you will be fine. You were planning on some extra wall studs to support such a long load bearing wall?