Snakehunter,
You really have a few choices on what to keep outdoors, it just depends on how much you want to spend on the enclosure, and how large you're able to make it. The species I'd suggest you can keep outside are the following, with recommendations on particular requirements each species will have.
Lace Monitor Varanus varius
These guys are big animals and need a large, sturdy enclosure. For an adult pair you're looking at minimum of 4m x 4m groud area, and about 2m high at least so they can climb, which they love to do. While they shouldn't require any supplemental heating, you will need to provide an enclosed area that is always dry and out of the wind, etc where the animals can take refuge. I would recommend though putting a heating device in there because I'd assume you'd get quite low temps out there during winter. This would only need to warm the "hotbox"/dry area to about 20-25 degrees. Lots of climbing trees should be provided, and they will sit in a large pond for short periods during hot summer days.
Sand Monitor Varanus goudlii
These guys are smaller than Lace Monitors and as such can be kept in smaller enclosures. About 2m x 2m but preferably larger with large gouldii. A good deep sandy substrate will serve the animals well as they love to dig, and this aids shedding also. These monitors will need supplemental heating and an enclosed area that is always dry and free from the temperature extremes. They will do well in an aviary type enclosure, but have a border of solid material around the bottom (like a kickboard almost) so they do not rub their faces away on the wire (which they will do).
Heath Monitor Varanus rosenbergi
These are great monitors, but very rare. Every that applies to the Lace Monitor applies here, although they are smaller, so just adjust the size of the enclosure. Also, they will dig more than climb.
Mertens Water Monitor Varanus mertensi
These are fantastic monitors, possibly my favourite I've kept. I keep a pair outside in Newcastle weather and with a few simple things provided, they have thrived. These animals must have a heated pond. Temperature should be set to 25/26 degrees. During summer this heating becomes less of a concern. I use a Stainless Steel Aqua One 300watt heater, inside a PVC tube wtih holes drilled so the monitors cannot come in contact with the heater. I use a black semi-circle pond, as can be purchased in the garden section of Bunnings. This seems a perfect size for the heater to work, them to bathe and me to clean. They will also require a hotbox all winter. They love to climb, and I use a mixture of Pine-Bark chip and woodchip as a substrate, which enables them to shed easily and looks great.
With all of these species, I would recommened enclosures with cage tops, not open pits. Simply because monitors are damn clever and escape easily, also it deletes the risk of feral animals (including the human kind) getting into the enclosure.
All enclosures should have a floor, be it mesh or concrete or otherwise. This floor should be able to drain well. I would not recommened any sharp wire (such as aviary wire) as the flooring for monitors that dig, particularly Sand Monitors as they will literally dig until their claws are gone.
Look into the diets of the animals you want and see what you can afford. Monitors generally do best when they are left alone, particularly in an outside environment. I found that the look-don't-touch method of keeping is quite successful. Others have success in handling but it depends on each animal and also the time you have available. Sometimes it depends on how much skin you have available ;-)
Of the four monitors above, the Sand Monitor is the only Class 1 reptile in NSW, to keep any of the others you will need a Class 2 licence.
Simon Archibald