I understand, and sometimes one has to feel the fear and do it anyway or something like this.... good name for a book
Before you say absolutely NO to land and house packages for whatever reason. I would meet with one of the sales reps or phone, find out what they do, what is included in their contract, how long to build etc, work backwards... if then you are not comfortable move on.
At the moment it is very difficult to source land, therefore perhaps it will be easier to go direct to the builder as they have their own contacts and have access to blocks that you now would not be able to secure.
Sash uses Collier Homes, because they have provided a good product at the right price. I also know some great builders but will cost you more, for this exercise you want cheaper to achieve the higher yield.
Here is a link from Collier, providing land and house for $410,000 in Wellard.
http://collierhomes.com.au/house-and-land-packages/chandon-park-estate-wellard/
I would also look at what this property/product would sell on completion.
Keep working it
Cheers
Marisa