That board will only work from about 3.7 to about 4.5 volts, so once again, WHAT KIND OF BATTERIES ARE YOU PLANNING TO USE???
You switch modes by cycling the light on and off quickly, which tells the microprocessor to switch to the next mode. Some boards remember the last mode that was set so you don't have to change it every time.
This board will try to push 1Amp of peak current when set to high, so it would be a disaster if the mode got accedentally switched to high for more than a couple seconds, if as you say, there is not going to be any heatsinking on the LED emitter in your light. To keep this from happening though you can easily remove two of the three 7135 chips from the board to create a lower current version (same multiple modes, but all modes reduced to 1/3). Then the MAX mode would only be 350mA and the medium mode and lower modes would have very long runtimes and no heat problems.
If you only want one mode, and can afford to run three or four 1.5 volt Alkaline batteries for power, you can get by with a simple resistor to limit the current, (so the DX driver you are looking at is more complicated than needed), but if you only have 1 or 2 NiMH cells you will need a boost driver, so this driver will not work at all.
