Harv- when speccing the suspension units, don't forget to account for the weight of the trailer! I know someone who did that ( cough! ) got a good supplier though & he took them back.
U2U sent. Not sure what type of finish you are considering but hot dip galvanze would be the way to go.
Have you thought about fabricating sub assemblies that could be hot dipped galvanized in bulk and then bolted together along with the lighting, tow hitch and suspension.
Even better, fabricate the sub assemblies and have them galvanized then sell them in kit form with advice on what suspension and tow hitch to buy and fit.