The most important thing to consider (aside from where you want to put the lamp and how much you want to spend) is to get a lens that will enlarge as much as possible at the correct focal distance for how you work.
The diopter of the magnifying lens refers to the power of the lens, i.e. how much it will enlarge. Focal length refers to the distance at which a lens will make an image appear to be in focus. So why do you need to know this? If you want to work on a doll head under a magnifier lamp, you do not want the sculpting tool or paint brush to hit the lamp. You must have enough space to work under the lens. To calculate the power of a lens needed to focus at a particular distance, first convert the distance you need work into centimeters, and then divide 100 cm by that number. The result is the dioptric power. Another way to determine that is to consult a chart. One is found here:
Clear as mud? Try taking a doll head and sculpting tool and placing it under various lens power lamps set up for display at an art store : )