www.gammon.com.au/breadboard
Go there, see how to make your own Duino CHEAP. Crystal is an option.
You still need to figure out how to code so that not a single cpu cycle is missed in the timing code. Gooooood luck!
I have a board from Slovakia made by Budvar (member here) that runs overclocked on a 24MHz crystal.
You can buy AVR chips guaranteed to 20MHz. Build one with a 20MHz crystal and your clock interval is 50usecs.