I have done a resto like this and it takes days of work. IMHO salvage the useful parts and the registration and rebuild from new steel. The new steel will Weld better than the old and you wont have to muck around fixing holes.
-measure up you material requirements
-hire a metal cuting drop saw for a day
-cut all your parts.
-Clean up with grinder and wire brush
-Take the whole trailer down to the local electroplater and have the whole thing hot dipped ( saves priming and painting)
-Then assemble ie springs axles woodwork
I would recommend Angle iron 65 x 65 5mm thick and 40x40x3mm box section
I would also make sure that you are confident you have the welding skills
"All that is necessary for evil to succeed is for good men to do nothing"
(Edmund Burke 1729-1797)