1/20" is fairly thin. This has advantages and disadvantages. The advantage is that it'll be less "effort" to work it (less material to drill/tap through). The disadvantage is, it's SO thin it'll be easy to distort and get a really poor thermal transfer beneath the LED.
Do you have a drill press? Have you done much metal work in the past?
As far as the drill and tap size, I'd assume it's coarse thread, i.e. M3 x .5mm. The nice thing about metric is, you don't need a drill size chart, you just subtract the pitch from the diameter and that's your drill size: 3 - .5 = 2.5, so you need a 2.5mm drill for the M3x.5 tap.
Are you totally in love with that screw? There are self tapping screws available in similar sizes which would eliminate the need for the tap. Can you use those screws without a tap? Probably, in soft aluminum, but SHOULD you? As mentioned above, material this thin is easy to mangle.