If you allow your animal to get acclimated pretty much on its own, it will be healthier in the long run. With in a few weeks, your monitor should be well on its way to being comfortable in its new surroundings, and should be beginning to feed well. Weight gain and growth will be obvious. Keep in mind, however, that some monitors, especially wild-caught ones, do not adjust well to captivity. They remain on the defensive all the time, and may fail to gain weight or grow much.
Many savannahs have trouble adapting to change.
If you get one from a private party rather than a pet store, expect the monitor to go through the same acclimation process. Once they are used to a routine, it is often difficult for them to get used to a new routine, especially when coupled with new people and different surroundings. In the long run, it is less expensive to buy an enclosure for your monitor to grow into, rather than to save some money and buy a small enclosure that will not last more than six months or so. You will still need to quickly start planning the enclosure it will be housed in when full grown. Stay away from open-mesh enclosures as these monitors must be kept warm and, unless you live in a consistently warm environment yourself, it will be costly and complicated to get such an enclosure heated to the proper temperature.