Hi Erik,
i would use a SILabs micro, just to get the beginner the advantage of the 'live' debugging.

Si Lab parts are not easily available here in India using Si parts will contradict

2. Uses parts that are readily available so that acquiring the necessary parts will not be difficult for those that wish to build it.

How about using P89V51RD2 easily available and feature rich.

1. 6 cycle core.
2. 1 KB RAM
3. 64 KB ROM
4. 40MHz operation.
5. SPI + UART.
6. PCA PWM
7. Internal WDT
8. In built brown out protection
9. Dual DPTR.
10. ALE Inhibit.
11. Easily available in local market for approx. USD5.

Regards,
Prahlad Purohit