To put it in a little perspective, It costs about $50 to $75 dollars a year to feed one ackie. A ball park of 3000 adult crickets and 100 pinkies. $45 or so for the crickets and $25 or so for the pinkies. Of course you can find those at higher prices and lower prices. So its an average. As well as amount, you can feed more or less, but that amount will work well.
Compare that to a larger monitor of any of those species. Is one rat a week enough, that would be 52 times, $6 or so, thats $312 bucks right there. Depending on the size, you could feed two to three times that. So if you have a large water monitor, you could easily pay near $1000 bucks a year to feed it. Again you could feed less or more or pay less or more, its ballpark U know.
Then take Electricity. One 45 watt flood or 56watt incandesent can heat an ackie cage, but it would take a bank of higher wattage bulbs for a larger monitor. On this I am guessing, $30 bucks a year for an ackie and several hundred for a large monitor.
How about the cost of a cage. I build my breeder ackie cages for $90 bucks a cage(four foot troughs with lexan lids) or less. It would cost many hundreds for a similar setup for a larger monitor. Lets see, a four foot trough is ONLY twice as long as a good sized ackie(2 foot) So to be equal( which is minimum to start with) You would need a 12 foot or longer cage for a larger monitor. A cage that size is expensive and has to be waterproof. Which is expensive.
All this and you do not have experience to deal with any monitor yet(or you would not be asking this type of question)
So I recomend a gouldi cross, as they are not expensive, they are manmade, so you are not impacting nature by taking wild caughts. And if you make a mistake and loss it, no big deal. I personally think you/we should be VERY responsible with wildcaughts. That is, we should be expected to do far more with them. As they are a product of nature and nature is growing thin.
Also gouldi crosses are hardy, don't bite unless they think your food and that will occur unless you teach them otherwise.
Of course ackies are great, as they are no so fast as some other smaller monitors. Which means, if you drop one, you may be able to catch it. If your drop a tristis or pilbara or kimberly outside, its bloody gone, like a rocket. Oh if healthy. Ackies are a little fast, but not too bad.
So please think about it, and pick something you can learn from and grow from there. Cheers