Insulation - Polyiso sheets. 1/2" is easiest to work with; cutting and bending. Just layer and glue (3M 77 spray) it in between the ribs. 1" in walls. 1.5" in ceiling should be plenty unless spending winters in Canada or Alaska. Insulating the floor in my opinion is optional. I would probably attach polyiso between the ribs under the trailer, to avoid building a raised floor inside...just glue your vinyl squares on the trailer floor.
Don't 'overbuild'...stay away from 2x4s and 3/4 plywood. Use 1x2s and 2x2s, 3/8" or 1/2" plywood.
File cabinets?--looks heavy.
You'll be surprised how fast weight builds up...unless you're going 'heavy duty', you want to keep it light. Gas ain't gettin any cheaper these days and weight costs mpg.