Once it was finished I just applied for a VIN, took it to a registered weigh bridge and paid for a certificate of Tare weight, applied all the details to the vin tag and it was ready for licence.
No engineer needed.
The cost once made was just the weigh bridge certificate, pit inspection then licence fees.
I know with my trailer it has 10" electric drums, so the max weight was 3000KG, if I had 12" electric drums it would have been 4500KG, the axles have parallel bearings, the tyres have a 1500kg load rating each and the leaf spring set up was rated to 1500kg per spring, it was just the brake drums that let me down.
They can base the maximum weight on axle type, spring rates and what tyre you use, so you cant just put a set of car tyres on it as they would not have a decent load rating, plus when you register the trailer they will ask for the specs of the components used ( brake spec, leaf spring spec braking capacity etc), worth having a talk to whoever will be licencing it in the end as they can give you the necessary paperwork for this.
Donor caravans don't come up that often and when they do its luck of the draw what they go for, the auction I use is one that I used to use for buying stock for my wrecking yard, so it was a Dealer only auction site, and as I am still registered, I still buy from them quite regularly, they are a Australia wide auction site, I was just on at the right time when this one came up, I didn't need it at the time but couldn't let it go at that price.
You would be better off building from scratch if you can, then you can make it suit what you need, this one just ticked the boxes of what I wanted, it wasn't planned, I purchased the caravan for the parts to make my slide on camper, the car trailer was just a bonus.
You can apply for a W.A. trailer VIN here.
https://www.wa.gov.au/service/transp...ication-number