Dan, don't sweat it, everybody is a lttle overwhelmed when they first find this site. There is a lot of new and somewhat confusing information to absorb. So, the best thing is relax and not be in a hurry (easier said than done, I know).
First, tell us a bit about how you want to use your new light. You say you want a focused beam, so you it sounds like you want to spot something far away?
Just so you understand, about 95% of our new members say the same thing when they come here. They want the brightest torch that will throw the longest distance :). It seems very impressive, and it is....... but ultimately it might not be as useful for everyday use, so I'd recommend that you think about how you want to use your light.
Do you want multiple levels? A single level drop-in module is usually less expensive, but sort of a one-trick-pony, while a multiple level drop-in allows you to tailor the amount of light to the individual task.
Look at the UI (user interface) for multi level drop-ins. Does the high come on first, or does the low come on first? It might make a difference to you. Do want a strobe?
LEDs come in different tints these days, from cool white, to neutral white, to warm white. The cool white drop-ins are generally the brighter, but the neutral or warm white drop-ins can be more pleasing to the eye, especially if used outdoors, because they tend to render browns, greens, and reds more faithfully.
If you want "ultimate" throw, then an aspherical lens is a relatively inexpensive way to achieve this.
You are right, the mAh rating is the capacity of the cell. Different drop-ins for your L2 are rated for different voltages, just match the voltage you plan on using to the drop-in.
Mainly, spend some time reading and absorbing, and don't be in a hurry. It takes time to put together the "perfect" torch, so look around and have fun.
