If you want to go with the 3/4" wall framing, I would add a 1/8" layer of plywood to the outside, and make sure both skins were well glued to the insulation and framing. This stiffens up the wall considerably. I'd have to crunch some numbers, but I would think the 3/4" sandwich construction would/might be as stiff as the 1 1/2" without the outside skin. (or using fiberglass batt insulation)
You could split the difference, as Joe mentioned, and go with 1" foam/framing. This is what I did for my trailer walls.
If you have 1 1/2" framing/insulation, and add the 1/8" plywood to the outside, under the alluminum skin, you would have the strongest walls of the mentioned choices. If you take a look at this thread
, you can see Captain Sam demonstrating the strength of the sandwich construction method.
Pro/Cons... Thicker is heavier, but not much. Thinner is less stiff, but can be compensated for with the sandwich construction method. Thicker is, uhm... thicker. Less interior space if you keep the same exterior dimensions.
Are you rebuilding the trailer, or just using the doner trailer for parts? The windows and doors will be probably be easier to use if you keep the wall thickness the same.
Insulation choice: I would go with extruded polystyrene insulation. Its the pink or blue stuff. People have mentioned that the white foil/fiberglass faced insulation may not be dimentionaly accurate. (or measure each sheet before you buy it, so you're not supprised)
Good luck, and welcome to the obsession.