You don't need a steered wheel if you have two independently driven rear wheels (differential drive). Just use a swivel caster for your third wheel. In my experience 100W motors may be a little small for a 100 kg load, but that depends on how quickly you want to be able to start and stop, surface, inclines, etc. I think they would be fine for a slow bot on a hard flat surface.
Also, for my yard robot, which easily handles a 100 kg load (e.g. my fat butt), I used wheelchair motors and constructed the frame from 1 1/4" square steel tubing (0.083" wall thickness).