you'll be able to source LEDs from various places. generally we'd suggest buying new LEDs– surplus LEDs are often unreliable and may die early, so keep extras.
you could also use surface mount LEDs– 1206 or 0805 work well. solder them on similarly to the diodes.
brightness: suggested 500mcd - 2000mcd
lower will be too dim, higher may create a bright “spot” in the middle of the pads. note that the kit has brightness control via software, and you could also switch the iset resistor on the driver to lower the brightness (but not increase brightness).
angle: as wide as possible!
a 20 degree angle may result in a bright spot. a 50 degree angle should uniformally light up the entire pad.
Vf and current: below 4.5V, 20ma
most LEDs fall into this range.
color: your choice.