Polycopters

Background reading

Android phone - free GPS/gyro/cam/Wifi, but long boot time - even with a separate watchdog, crashed phone = crashed copter. Still need to break out an interface to motor control board.
Arduino 2009 / UNO - nice hardware as has A+D I/O built in + neat shields for motor control. GPS/gyro/cam/control method will still need to be worked out. Memory space too limited for full autonomy?
Arduino Mega - as above, more expensive but more memory, enough for full auto?

Barometer - least sensitive, probably best for high-flying vehicles. Needs to be kept away from the wash from the propellers.
Ultrasound sonar - very sensitive for low-level flight, but tops out at a few metres.

"What do we need to have in order to do x", in increasing level of unlikelihood that our vehicle has it:

3 axis Gyro - required for AP behaviours: takeoff, land, hover, up, down, move wrt vehicle orientation, pitch, roll
2/3 axis magnetometer/accelerometer - required for AP behaviours: move wrt north; local time-based autonomy behaviours (think LOGO)
Barometer / ground-aimed sonar - required for AP behaviours: set altitude
GPS - required for all outdoor location-based Autonomy behaviours, accuracy ~10m
Multi-band RF receiver - required for all indoor location-based Autonomy behaviours (triangulation)
Heading unit - negates need for magnetometer if in GPS coverage
RTK GPS - improves position accuracy in heavy GPS coverage to ~1m
RTCM GPS - improves relative position accuracy while in communication with static base station to ~1cm
Outward-aimed sonar - required for Autonomy behaviour: dynamic avoid
Radar - required for Autonomy behavior: dynamic rendezvous (allows dynamic avoid too)