Firstly find an architect/Draftsperson or hand holder to help you through this process.
I recently met the team at Pillar and Post (Christianb and Breece are members of this forum) and the are great at holding a beginners hand through the process of due diligence and working out what will fit on a block.
The problem is, you haven't told us what suburb, what zoning, are there overlays on the land, where is the sewerage, storm water, etc on the site. What is nearby? Are there any covenants on the block such as single dwellings in that suburb only.
As for how much, as a percentage profit, I don't normally look at deals (for development) under 25% return, the risk of hitting snags and issues doesn't make it worth it. I know experienced developers who develop in the same suburb over and over work at 15-17%.
How much to build? How long is a piece of string? IS the block level? What finish are we talking, 25 sq is a big townhouse, does it have to be that big? Double storey or single? the list goes on.
You can do development, you just need to go in with your eyes wide open and knowing everything you can about it first, and don't rush! There is always another deal!!
Hope this helps!
D