We built, but did house and land so its a bit different

290K is good if it includes sitecosts - make sure this is not capped. Sometimes sitecosts can be tens of thousands above the estimate - has the land been surveyed as yet? The site costs for our house were fixed in the price but were over 30K on their own.

You also need to think of costs of things such as TV antenna, gas connection, electricity connection, phone connection - these can be between 150-300+ each
Landscaping can run in to thousands depending on what you want and how much you can do yourself. We havent been able to afford to get ours donw yet other than the front which was included, but there are no plants so far.
Do you want a water tank installed?
What sort of heating annd cooling - ducted heating and refrig cooling will cost 5K-7K
Are your kitchen appliances and cabinetry included?
Letterbox

As far as mortgage goes, you can get a progressive drawdown loan where you pay in stages, and you only pay the interest component until the last drawdown of funds, then you start paying the full amount.

If you think 30K will cover it, count on it being 40-50K