I'm a police officer, I got annoyed with no "issue" torches being available on busy nights so I bought my own torch. The station issues mag rechargeables. I bought a Led Lenser X14. I've been doing this work for about 7 years now, so I was pretty sure I knew what I needed. And I LOVE this thing.
It easily fits into your back pocket when you get out of the car (it came with a belt clip, but I'm not that keen on it).
It runs on AA batteries (always available at the station, but not expensive if I was to buy my own).
You have the choice between energy saving mode and constant current (constant is great, stays bright throughout the battery life).
You can focus the beam to a brilliant spot if you want to illuminate a person out to a couple hundred meters, no problems!
You can adjust the beam to flood, I find this is outstanding in dark rooms (even very large rooms, warehouse sheds etc.)
You can adjust the brightness to low with a press of the button, this is perfect for reading/writing at the side of the road at night, or entering yards when you don't want to disturb neighbors, or going into small spaces to search.
The beam is bright enough to easily view what's inside a car with tinted windows (important!), and powerful enough to disorient an offender (emergency only, of course...).
I haven't made any effort to time how long the batteries will last, but you get a series of 3 flickers to warn you well before a change of batteries is needed. A spare 4 AAs is very easily carried anywhere.
Oh, and it has a strobe/self defense function. I thought this was a bit odd and maybe unnecessary at first, but I have found it EXCELLENT to keep large dogs away from me (I shan't guarantee it works on every dog, but so far, so good).
I only use the spot and flood at both ends of the possible focus (haven't found a necessity for anything in-between), and it's very easily done, even with one hand.
The thing seems solid, it's been dropped on cement/asphalt without anything bad happening to it. Don't be put off by its appearance, in my opinion this is a great torch for work. I won't comment on anything technical, because I've not much idea about lumens/lux/voltage etc, I'm just an end-product user. I imagine the rechargeable version would be very good too, it appears a little smaller but I've never used one.
I won't address the issue of mounting a torch on a weapon, I suggest you don't worry about that until you go to a specialist unit.
Hope this helps.....