buy 1 of each 25w, 40w, 60w incandescent bulbs.
As a baby they will not need that much temperature and depending on where you place the basking branch and laying the light housing on the screen you could most likely get by with the 25w.
As they become juveniles and getting older you could switch up to the 40w bulb and increase the temps slightly by doing so.
When they become adults you will most likely be able to run the 60w bulb at that time.
Granted this is all dependent on how far the branch is from the light etc...