One thing I thought I would mention, is look into what your builder means by their 18 week build guarantee. We are currently building and our neighbour is going with a company that has a similar building guarantee, however, the guarantee had lots of 'conditions'. For example, the 18 weeks didn't start until the house had council approval (which can be several months from when you buy the block of land, which you will will need to be making repayments on in the meantime) and their build time also can be extended due to 'bad weather' (or whatever the builder deems to be bad weather). There are other conditions too but I can't remember at the moment.

Our builder doesn't have such a guarantee, but our house is taking a 'typical' amount of time based on most houses to build in SA these days. We signed up for our block of land and with our builder last August and our house will be ready in a months time (13ish months total). We had to sell our house and rent as there was no way we could afford our repayments on our other house as well as repayments on our new house.

Good luck!