I've been looking at these two boards:
http://www.icoptech.com/products_det...?ProductID=277
http://www.icoptech.com/products_detail.asp?ProductID=300
The big plus on these two boards is they have a GPIO that can be used for external devices.
I'd like to second the idea of using FreeDOS. It's small and there are a lot of tools that will work with it. It's also easy to write drivers, which will be a plus for the external devices you are trying to add.
You said you were looking for a Unix alternative, you can look at
Minix. There are ports to X86 boards. It's also small and there have been ports of a number of tools to it.