Hatchling/juvenile wouldn't need an absolute heap of room for quite some time... Adults do get reasonably sized though, ~1m long.
There's a book called "A Guide to Australian Monitors in Captivity". While not encyclopaedic in its content, it's pretty bloody useful in terms of species-specific information, husbandry requirements, health and nutrition. Depending where you shop it's roughly $70 but worth getting.
[MENTION=40362]CrazyNut[/MENTION] I'm in the same boat dude. Strong frame topped with form ply, build it up from there. Either a hacked-up water tank with a lid or aviary-type frame on the base would be the way to go for us renters. Whack it on castors and you can wheel it into the garage in winter or down the street when you move house.