Dragan,
I have been using a 5x8 since 2004 and works very well. I have not tried insulating it but have though about it. I would guess using some kind of spray urethane foam for the job. That is assuming you will vacate it in the event of fire. You have made a good choice in the torsion axles. Also make sure they are genuine "Dexter" axles because of future parts availability. Should you ever need them. If you get one with the roof vent, you will need some upgrade there from the standard crank it up type. The covers don't seem to stay on at interstate speeds. I use a hooded like vent that fastens on top. That has served me well? in speeds in the upper 2 digits and the occasional jaunt? into low 3 digits. That brings up the aerodynamics of the front. I have a rounded type that needed some reinforcement. I just added in shelves? coped to fit the front to act a bracing. Now the front end stays round at travel speed. The V-type may be more robust. Ramp door in the back is worth the upgrade and save a lot of time. Don't scrimp on tires. If you plan on hauls long distances, say 1000 miles or more,? just plan on getting fresh tires a few years. Trailer tires seem to dry rot a faster pace the normal tires for some reason. If the tires are made in China, don't inflate them at the stamped pressure. If they say 65psi I run at say 55psi and they don't pop nearly as often. If I think of anything more I will post .
Have fun,
Bob r
bobrose500