Go the 5.7L. you will have to spend a lot of money on a 5L to get it to keep up with a 5.7L. in power and economy. The 5L will be harder to sell when the time comes, as they are known as an orphan in the motor industry (no-body wants them).
If you are in Melbourne do as recommended by "justbad4you" and check out Autogroup. I was there 2-3 months ago and they had a blue 04 SS with 60K on the clock (Ex-Police, immaculate) that sold for just over $30K, not sure if it was a VY or VZ it had those fake vent/scoop things on the front guards if that is any clue.
I would think, that as sales of V8's have stalled (fuel price rises) your money should go further now than it would have a year ago, (Xfordman got one for 15K). I personally think it will all blow over, consumers will get used to paying the price and will drift back to the larger cars/4WD's. Thats what happened in the late 1970's early 80's, I remember seeing 350 HQ Statesmans in Kevin Dennis newspaper Ads for cheaper than corollas and datsun 180B's, a few years later the prices bounced back.