Iirc the max width to be legal everywhere in the US is 102"
With only a 16' deck you're nowhere near the length limit.
No max weight empty, though staying under 2,500 lbs empty costs less for the plate plus you don't have to get a title ($15). The bottom of this page lists MI Trailer and Trailer Coach Plate Fees: http://www.michigan.gov/sos/0,1607,7...5432--,00.html
Toward the end of this PDF it lists trailers and their light and reflector requirements.
Another with diagrams of where the lights go.
Never heard of any legal restriction against using torsion axles. You just have to tow the trailer level since there is no equalizer to help balance the load between the axles. How much are they rated for? You'll need brakes.
What GVW are you shooting for?