First you should consider what type of range you need to get?  Will a 6v system which will give you about 50-60yds. of clear target identification be enough or do you need to go with a 9v system and reach out a bit farther to ranges of approx. 80yds.?

If you're trying to save every dime possible but still like to have upgradeability I think a 6v M95x SF Millennium system would be a great start.  You can take a few shortcuts and save some money by going with a Z48 clicker tailcap and bypassing the ARMS mount for now by using a std., inexpensive scope ring to mount the system -

Quoted: I would prefer the more power.  Where is the best place to go to get a good deal on a light with tail cap?

Well, I'm an authorized SureFire dealer so I can certainly help.  Or, there are other SF dealers here and you might even have one locally.

That person wanted to see it mounted as far forward as possible and he also wasn't ready for the expense of the ARMS base.