1st you have to ask yourself how much you want to put into this trailer build: time & money. How far are you willing to go as of materials used (all metal constructed or metal & wood?). Cheap IMO isn't the way to go for a offroad trailer.
Every state varies in registering a DIY trailer. I had zero problem doing so. Arkansas require length width & empty weight & as well as gross weight. That's it they issue me a VIN number you follow the instructions & the location of the VIN number & have it stamped on a plate & welded or riveted & or etched\engraved on the tongue rail. I insured mine for 10,500. & had no problem doing so.
I built my overland trailer M416 style w\RTT. Chassis & suspension is important factor. I went with offroad suspension w\shock kit. My chassis can handle the riggers of offroading in rough country. My axle is a 2000lb axle. Tongue length is another to think about My tongue is 6" shy of my chassis length which is 6ft long frame, 5 1\2ft. long tongue. My complete build cost me (trailer: chassis & tub, RTT, awning, primer paint, tires wheels, fenders, bed liner,tonneau cover, jacks, solar system) 7,400.00 w\o camping gear. Time frame for my build was 9 1\2 months every week end & lots of nights during the week days. This trailer is very structurally sound.
I thought about building a tear drop @ the of this end of the year. By the time I figure material cost, it would exceed over what I can by a bare bones 5ftw.x8ftl.x 4fth. tear drop that is design for offroad & constructed of all 100% aluminum frame, 100% aluminum wall framing, 100% aluminum ceiling framing, 100% aluminum sleeting exterior walls, roof & floor. This includes 31\10.5\ 15 tires & rims under 7,000.00
Lastly do you want this trailer to be problem free: structurally & able to go off roading?