If you have little to no experience cutting and fabricating metal, then adapting a utility or used boat/camper trailer may be less feasible for you. I'm not saying that it can't be done, nor that you can't learn these techniques, you can, but it does require special tools and PPE (safety apparel), and takes some experience (or guidance from someone with experience to learn safely) that will cost you time and/or money to achieve.
Used boat and/or camper trailers will also most likely require modifications to existing cabin designs, and/or cabins that are specifically designed by you to fit (...and still look good while doing so...) due to existing dimensions, xmbr locations, fender and wheel placement, tongue configurations, etc.
IMO the bolt together trailer kits are the best starting point for people that have basic wood working skills, but aren't as comfortable with metal fabrication. Usually the only modifications needed only require repositioning a few parts, drilling some new holes, and maybe making a couple of simple cuts/notches. Extending the tongue, which is highly recommended, is also not too involved. These modifications have been well documented by many people here and can usually be done w/o the need to weld.
It is not my intent to steer you away from any particular choice, but hopefully my perspective can help you evaluate your options. If you are a fabricating mad women, or wish to learn, I'd encourage you to build from scratch. If you are handy and want a challenge, but aren't quite ready to take on the challenges and time commitment needed to build from scratch, I'd say modify and/or restore an existing trailer. If you just want the trailer part to go together relatively easily for not a lot of money so that you can get on with building the box and go camping, I'd say buy the kit or a used one on the market (they come up quite frequently in my region).
I hope this helps you to decide, and I look forward to following along, whichever method you choose.
