They all typically run on the 3.7 typical volts...so any watch battery is decent. Also they have multi intensitys depending on the viewing angle...I would use like 15-18 degree for head lights, and maybe 30 degree LEDs for a neon underglow...remember from the real customizers world...if you can see the source of the light from the glow...you lose.
binaryterror, those leds are a little big. Even with the 3mm leds, I couldn't get them to fit right. Underglow is a little difficult to get right on a bit.
d00kie, yes RS is expensive for white/blue leds. White/blue leds are the most expensive leds. Have you tried Wedginator's link? That's a really good price for white/blue leds.