It looks like I've completely overlooked the fact that I can only connect 1 or 2 expansion boards (Basix or Connex respectively) plus a tweener for RS232.
For an Autonomous Air Vehicle:
I'll need a wireless serial unit for comms (whose interface I am assuming will be an RS232).
I'll need analogue (and probably digital) I/O for sensor inputs and filtering, and servo control. I wanted to use a robostix (Hirose 60-pin)
I'll need GPS. I wanted to use a GPSStix (Hirose 60-pin, but FAQs talk about connection via a STUART as on the RoboStix!).
I wanted MMC for memory expansion. I wanted to use a Basix for its MMC.