Thanks a lot for all replies.
I see that the I/O situation is much better then i thought, even using SPI.
A special thank to point me to 'ports'. The information about that is very well hidden on www.arduino.cc (I could not find that on my own, not even with your tip. Had to ask google).
See Arduino Reference - Arduino Reference
OK, now I wait for my hardware to arrive before asking any more questions...