I am not sure why you are "imagining" stamp duty. Stamp duty varies from state to state. Look it up for your state and no need to "imagine" anything. For example: google "stamp duty"+victoria That'll take you to the website with the stamp duty figures for Victoria. If you live somewhere else, google their site.
As for "additional costs": That's a case of how long is a piece of string. Do a search for threads on "budget" and "pre-start". As a very general rule of thumb, allow an extra 10-20% of the house base price.