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.
Savannahs have incredibly sharp claws, and can easily shred a hole in screen.
Make sure that the walls, floor and ceiling are securely attached to each other. If the savannah finds a weak spot, it will work at it and work at it until it works a hole just big enough for it to squeeze through. Along the same lines, keep the enclosure away from drapes, expensive lamps, computer equipment, etc. When taken out of it's enclosure, savannahs will scrabble around trying to hook their claws into anything they can. The monitors found at the extreme south of the range experience cooler weather.