I cut out the opening for the main light (its going to have 3 different lights) with my jigsaw, its not perfect but you won't be able to tell once the light is mounted.
I measured and ordered two straight 5200lb axles (with brakes on both) from a local trailer dealer, when they came in they were 6000lb, he let me have them for the same price. I welded the hangers on with the trailer upside down then flipped it over and bolted on the axles.
Trailer is skinned. I used 14 gauge diamond plate for the decking. I used five 4x8 sheets. The cuts were made with both a cut-off wheel and my jigsaw. I used a whole tank of shielding gas welding the plate on. I used 1/2" metal for spacers on the cross members to level the plate to the outside frame.