Best expandable Arduino starter kit

Quicksand10:

  1. Will I be able to follow, for example, an Uno guide if I have an ADK Mega? (I'm guessing not always vice versa?)

Yes. "The ADK is designed to be compatible with most shields designed for the Uno, Diecimila or Duemilanove. Digital pins 0 to 13 (and the adjacent AREF and GND pins), analog inputs 0 to 5, the power header, and ICSP header are all in equivalent locations."

  1. I am considering adding an Ethernet shield (or preferably a Wireless-N shield, if you guys say they're okay, not b/g -> will downgrade the network from n to b/g), so the Arduino receives its commands through the network. This is possible without running another server connected to the Arduino by USB, right?
    2a) If yes, Thoughts on this? http://www.dfrobot.com/index.php?route=product/product&product_id=548

Sorry not familiar with that shield nor have I tested WiFi. However, I have been using dfrobots bluetooth module for sending commands without usb, so I see no reason you couldn't do the same with WiFi. If you are wanting to upload sketches though, I don't beleive thats possible with the Ethernet shield or WiFi shield.

  1. Any suggestions on documentation for me to start with? (is there different documentation between the Uno or the ADK Mega, appart from the 'ADK' documentation from Google?)

Other than Google, I don't know of another central location for adk, but you should look two URLs over.
http://arduino.cc/en/Main/ArduinoBoardADK
http://labs.arduino.cc/ADK/Index