If your looking at a loan, think of a few things. The longer you take the loan over the better credit history you will get for it, also it'l be cheaper per week, and if you wish to pay more off it than the normal weekly payment you can always do so, and in this case shorten the loan term, or the weekly deduction.
You'll have a hard time extending the period of a loan if you go for an unrealistic target and find yourself unable to make payments.
For example, a personal loan for $20,000 with the Commonwealth Bank of Australia at the moment + interest of $6,852.80 over a period of 4 years works out at $258.20 a fortnight. Which is not too bad, and certainly leaves you some $$$ to play around with out of the pay packet.
I purchased my 96' R33 GTS-t with funds saved over a period of years and I'm 18, but I did look into quite a few loan options. Also the Commonwealth was happy to go ahead with this loan (was looking at a GTR at first) and I had no previous credit history, however lived at the same address for 16 years, and held my current job for 2 years.
A few questions need to be raised however, are you currently in a full time position ? Studying ? Whats the deal ?
Living at home ? Moved out ? Moving out in the future ?
Does your current wage allow you to ditch the payments and still live in the style your accustomed to ?
Hope this helps