Thankyou again for your replies.
I have spent the last 2 weeks building my new trailer. Little bit at a time, and all of the top of my head, and from previous trailer ideas. I used 3mm on main draw bar, and 2.5mm on the rest all in duragal. I ground away the areas to be welded and used a Mig to weld. I have oil filled all crossmembers, and used linseed to fill them, incase of leakage, to be environmentally friendly. I probably went bit overboard with my joins/design and extra angled corners etc, But id rather it over engineered, as its carrying a reasonable size tinny. I re used the existing winch post,with a few mods. Just waiting on a new axle arriving via couriers, then have new guards,hot gal dipped wheels, and all new rollers. I was able to clean and re use most of the brackets and a few other peices. Im going to run a 12000lb 12v winch, and also i have welded a bracket to hold a manual winch as back up. To protect it, I have zinc coated the welds,I am etch priming the whole thing, then using Ormonoids bitumen over the top. Its in Black, and depending how it looks,it can stay black. If i dont like the finish, ill use ormonoids silver gal over the top. I will also make a irrigation hose set up, and hard mount it inside my 2 full length drawbars, with a tap fitting, so i can easily flush inside the steel, thats not oil filled. hopefully this system holds up a good few years for me, and with regular washing, and regular re application of the bitumen, im confident it will do what i need. Ive built it for around the $800 mark now, and all i need is the 12v winch and rego to finish. Not the best pics, but you get the idea.