Remember to do the proper land calculation..
Estimate your minimum acceptable profit, estimate the value of the house at the time of completion (factoring in estimate of market chnage between now and then) and estimate cost of build, including interest on borrowings or interest foregone on own money used for buying land and funding build.

Future value less min profit and less cost of build =
maximum you should pay for the land.
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PS comparing new build to older houses to older houses, Mello Roos etc can be put in a comparison by dividing MR pa by mortgage rate divided by 100 ( eg 0.055)