Dark Sensing LED Light
I expect that the supply voltage is not very critical - 6 volts should be fine.
You can determine using Ohm's Law, that the maximum current through R2 will be about .05 mA.
An LED advertised as "5 volt" will be a bare LED with a suitable resistor or current limiting circuit so that it can be operated from 5 volts with no additional current limiting components.  (I believe the highest voltage bare LEDs are blue (or white, which is just blue with a white phosphor) which have a forward voltage of 3.0 - 3.5 volts)