The amount of infrared energy (a.k.a. heat) that can be focused to a point depends mostly upon the surface area of the magnifier lens, not its magnification power. You can have two magnifiers with the same surface area and different powers, and you will have identical results… the only difference is that you will hold one magnifier closer and one farther away when focusing the point of light.

The best bet is to get a good acrylic Fresnel (flat, with the visible concentric rings) magnifier in the biggest size (surface area) that your desired kit will accommodate.