I get the scenario repeated time and time again… singles, couples, families encumber their cashflow significantly with a shiny new runabout that serves more the ego and the lust than the function, and THEN see what their options are for a home loan. Have a look at a couple scenarios:
1 – A single guy on $75,000 with a $5,000 limit credit card who can normally borrow around $450,000 goes and gets that 50k BMW to prove to himself and his mates that he has ‘arrived’ and takes out a 40k loan for it, has just reduced his home loan capacity to $314,000. Not many choices here! Plus the new wheels are going to depreciate and not really serve any positive financial purpose in the long term. Let’s look at another scenario:
2 – A family with 2 incomes & 2 kids. Dad is on 80k (full time) Mum is on 30k (casual). The current family car is a bit dated but fully paid 
off and running well. All the new car ads on the telly keep bombarding them and they decide to test drive a new 60k 4WD. They get 5k trade in for the old car and now have a 55k vehicle finance. PRE vehicle finance, their new home capacity is $581,000. POST vehicle finance they can now borrow $399,000. See the big difference? What this will mean ultimately is compromise in LOCATION (further away from kids schools, work etc) or worst case scenario not be able to anything at all. But hey, at least they have a nice car to drive that extra distance.
The moral of the story? If you have a car that works, or your transport situation is not weighing you down dramatically, get your home loan sorted FIRST. When your new home is built, get used to the repayments, rates and insurances you will now have. Once you have settled into your new mortgage paradigm, then and only then seek out your options for your next car. At least now you will own an APPRECIATING asset that may allow you a more secure future and maybe be pivotal in one day acquiring a car that you may believe is outside of your financial capability right now.