Well, with bike lighting in general, it really depends on the conditions and speeds involved. I'm guessing this bike will be urban and lower speeds, so may not need the high-power lighting of higher-speed/off-road riding.
However, more light can be useful to compensate for the "swamping" that passing and oncoming cars create. Also, riding between streetlights causes the same effects, though that light isn't moving and better illuminates the roadway.
The general rule of thumb here - no one complains about too much light.
That said, I'm comfortable with an R-bin Lux 1 driven at 500 mA. It's using an IMS-20 reflector, which gives a bright spot with decent spill. Aiming the spot to hit the road around 15-20 feet away fits my commuting speeds just fine, and as a "be seen" light, well, trust me, it's visible.
I've also got a T-bin Lux 3, driven at 700 mA, in a headlamp. The optic gives around a 6 degree spot, with less spill than the reflector. This is brighter than the Lux 1, but the lack of spill makes it less useful in urban settings. It will get a driver's attention, but if it were bar-mounted, I'd change optics quickly.
Having said all that, give it a shot - that's the best way to tell if a setup is adequate. My preference would be two separate LEDs, maybe 3 - the K2s are cheaper, and 3 at 350 mA should be more efficient than 2 at 500 mA. Multiple LEDs also gives you redundancy, and allows you to aim them separately, for a wider field of view.
Best,
Jon
