I'm not an accountant but I would have thought that to collect GST you'll need to be registered for GST?
Your builder will pay GST on the materials he'll buy and will get tax credits etc, he will charge you GST for the new house he's going to build for you
but I don't believe that you can pass on that GST cost to your buyer.
So IMO if you agreed to sell the property for $250K thats' it
but since we are on the issue of tax, have you considered CGT??