Having gone through all this with my Sleeping Car, my most important piece of advice that I can offer is to talk to the council FIRST (and second and third, to get the picture straight) before you spend a single cent.
The Australian Building Code was re-written in 2013, so while there are plenty of examples around of carriages converted as offices and accomodation, how many have been done in the last 5 years?
In NSW, a carriage does not fall into any
approved building construction class, and is listed as a Prohibited Construction. This is not as grim as it sounds, it just means that the whole project from go to whoa needs to be checked and signed off by a structural engineer and Council Inspector, again and again and over and over. There are restrictions and prohibitions for certain land zonings and lot size for a Carriage, and if you intend to let it as B&B or Backpacker use (ie commercial return), there are also other implications.
If the carriage has not had a DA and Occupancy Certificate issued by the Council to sign off on it, you will not get Insurance for coverage of loss, or liability to users.
If you just decide to plonk it there without any Council Approval or Insurance, realistically expect that "someone" will find out about it sooner or later, and Council will tell you to pack it up and get rid of it.
On top of the purchase price and delivery, your site costs are dependant on your site, but the compliance and engineering costs you WILL need to pay will be around $30,000.