Well personally, I think that your should start small with your first build and use regular disposable D cells, or Nimh cells, as these are low maintenance, low voltage, safe, and easy to use. My first Mag mod was a direct driven 3D P7, meaning that there was no circuitry between the batteries and the LED. Then I moved on to more complicated Mag mods involving dual drivers and multiple LED's etc.....
Anyway, if you insist on using Li-Ions, how exactly do you plan on using just one Li-Ion cell in a Maglite? Two 18650's can fit inside a 2C Mag (barely).