linux sbc recommendations
Not sure if it is quite a sbc, but the guruplug server plus has usb, 2x ethernet, uart, i2c, spi, jtag (with an adapter), microsd, no gfx, no ps/2, serial port booting (with an adapter) and runs linux from flash on an arm core. Maybe take a look at it ? With minimal trickery you could run it off a DC supply. They run for US$129.