My vote is for Atmel since it is easily available here in my area. I hope this is true for other parts of world.

Keeping in view that this board is for beginners, two relays, two LEDs, one buzzer and a 4 way DIP switch should be there. May be you can leave provision for an ADC, MAX1232, a seven segment display and a stepper motor driver chip?

What about PCB? You said beginners/hobbyists are supposed to build it. Do you plan to publish the design of PCB or it would just be a tested schematic diagram?