Over a long period,I have built six small trailers for motor cycles and stationary engines,plus trips to the dump.Some of my trips have been over hundreds of miles when buying stationary engines,My last home built trailer now 30 years old has had some updates,I based the chassis on those that indespension used to sell ie 6 ft x4ft ,measured up one down our local trailer centre when no one was looking,modified it by making it 6 inches longer and just over 4ft wide so that a new sheet of 6 x 4 sheet timber will fit inside and hang over the tailgate.It is unbraked I first used mini hubs and wheels and 500 kilo indespension units,first mod was to fit metro taper wheel bearings in the mini hubs which made the trailer roll better,and replaced two sets of indespension units due to bent axle shafts, the mini hub is only 7/8 ins dia and theindespension steel is not as strong as a Mini Spindle, All due to overloading with stationary engines and a 15 cwt surface grinder. The trailer now has Avonride units which have stronger one inch dia spindles,and trailer parts suppliers now have hubs to suit these axles and a variety of wheel sizes,try to get better quality bearings ,the real cheapo ones are not very good. The avon rides 550 Kilo capacityalso have solid arms from the rubber down to the axle and are a lot stronger than those made from rectangular tube.Try looking at Trailertek website. I would just build the trailer,stamp/mark on the max gross weight ,just guess it dont worry about weighing it, the gross weight has been mandatory for a very long time,also a safety chain is mandatory on unbraked trailers.nobody can tell how old it is so dont worry about latest regs which currently apply to manufacturers. Make the trailer neat and tidy,paint it and add plenty of tie down points,trailers only seem to get stopped by the law if they have poorly secured loads and look as though they will fall to bits. I carry stationary engines to rallies so need reliability so use 8 ply tyres ,carry a good spare wheel ,plus a new wheel hub and bearing.and jack,dont rely on the car jack as a lot of cars dont have jacks, my wifes Vitara had as standard the aerosol of glue in the boot in case of punctures, no jack or wheel brace, so I ordered the skinny get you home kit,the kit comprised the wheel and tyre,plus nut spanner and jack ,then when I checked the kit the wheel nut spanner did not fit,wrong size so the suzuki garage had to rob one from their stock of cars oops.and then made sure the skinny wheel fitted, What made me flaming mad was the agent told me that a full size spare would not fit in the boot well, so I checked it out when it was too late,the full size spare does fit with about a 2 inch loss of boot space.If building a trailer costs too much,try finding a trailer of known make and make sure that it is not been abused,rusty,and does not have perished tyres,If you do consider making a trailer to the full 750 kilo capacity I think you will find that 750 kilo spring units will beat the load to depth as these rubber sprung units will take well over their rated load. By the way for my larger engines up to 3/4 ton an Ifor williams with leaf springs is the best ,these springs give a really nice ride for the valuable load. The problem with an Ifor is theft prevention.