Hi Andy, Welcome to the forum and greetings from Scotland. I hope you find something to build that will meet all your requirements.
As a start, I would recommend having a look at the
design library and/or starting a new post titled something like "What trailer should I build?" You will soon be served with suggestions.
As well as weight, width seems to attract a lot of debate. I avoided the temptation of being limited by an 8'x4' sheet so that my trailer could take a UK double mattress.
I'd like to finish by saying that the cost in the UK is *much* more expensive than you might think. I've totalled up what I have spent below, not to put you off, but so you go into this with your eyes (and pockets) open. Compiling this list brought me out in a cold sweat. My before starting gu-estimate was 600 quid. haha!
My fairly accurate, actual spend in pounds:
400.00 for Chassis: Steel, wheels, lights, hitch, etc,.
60.00 for PU Glue. (sixty quid on flippin glue!)
40.00 Epoxy.
100.00 Wood, Ply.
300.00 Aluminium coil, sheet and mouldings.
100.00 Screws, Bolts, Hinges, Wheel-arches, odds and sods.
100.00 Caravan Door Locks, Roof vent, latches.
1100.00 Total.....gulp! Pass me the smelling salts.
This does not include the cost of interior finishing or kitchen.
You might want to consider that you could buy a used teardrop from eBay for not much more.
Have a look at Dave's (Jackdaw's) trailers for inspiration. They are particularly fine examples.
If I haven't put you off, I recommend you go for it. It's been one of the most rewarding things I have done in years. :D