If you want to take the leap and transition over to the rechargeable lithium battery realm, do yourself a favor and just save up and do it. The cost of rechargeable lithium cells is comparable or slightly more than buying the name brand disposable batteries. The benefit is that you can recharge them up to 1000 times before replacement.
I've read and researched over on candlepowerforums until I couldn't read any more, but the reader's digest version is this:
Buy AW brand (protected) rechargeable lithium batteries - they have the best reputation out there by far.
Li-Ion Protected Batteries
Look at a Pila charger and nothing else -
Pila IBC Charger for 14500 / 17500 / 18500 / 18650 3.7 volt Lithium Battery Charger
You'll need 2 of these to go with your charger for charging the RCR123a batteries -
15mm Charging Spacer Aluminum
As far as lights go, your budget is the limiting factor. I will tell you this, the days of needing to carry a big mag lite to get illumination are OVER. There are plenty of small pocket lights (torches) that will get it done in spades.
I have 3 of these and they get it done and have a lifetime warranty. They are totally user customizable so you have the 4 settings you use most available -
HDS Systems: EDC Clicky Flashlights, EDC Custom Flashlight, EDC Executive, EDC Tactical, EDC LE, EDC Rotary, Ra Clicky, EDC-120 Recommend getting the 200 lumen models = most lumens and most efficient on batteries.
Honestly, for most day to day tasks, these small lights are all a person needs. Most often I don't even use the high setting, it just isn't needed for most things.
Nitecore is another option that is less money, but also takes rechargables and throws more light than the HDS. My wife carries this one in her purse and loves it. For it's size, it puts out a lot of light (800 lumens).
NITECORE
Available here -
Nitecore MT26 XM-L U2
If you must have something that resembles the size of your mag lite, here's a 860 lumen nitecore -
Nitecore MT40 XM-L U2 LED Flashlight
If you do decide to go the rechargeable route, think about having 1 battery or set if it takes more than one in the light, and at least one spare battery or set on hand that you can just swap out while the drained set is in the charger.
I can honestly say that I've never drained a set of batteries during an outing or work shift. In fact I can go weeks before I just swap out and recharge. However, I always keep a backup set with me, or a set of throw aways in my gear just in case.
I'm sure the other brands mentioned are good too, but HDS and Nitecore are the two I have experience with and both brands have been very dependable for me. If it matters, the HDS also work as a weapons light - I have one on my AR, so they can take a beating.