the City Auto can only tow 600kg Braked Trailer,If that is what it says, that is your lot, does not matter how strong the towbar is. 600kg does seem low. You should have this information in the vehicle specs book that came with the vehicle. Be very careful that you get the correct answer to this question.
The rated towing capacity includes everything.
All trailers over 749kg need brakes, irrespective of vehicle capacity. A light weight camper or van (under 750kg) might not be fitted with brakes.
After you have loaded the trailer, you can weigh the tow ball weight with some bathroom scales to get a good idea. There is a minimum tow ball weight as well as a maximum, depending on the weight of the trailer. You will need to go to a weighbridge to get the weight on the wheels. Add the two together to get the Gross Trailer Weight.
Contact your local vehicle standards / inspection station for the local State regulations.
I think towing anything at 80-100kph is too much, no matter what the vehicle is. Check your local State regs. You might find that there is a speed limit.
Anyway, you are on holiday. Slow down, and smell the roses!