Need a baromer with the GPS?
Mini-uino for light weight weight? http://arduino.cc/forum/index.php/topic,69622.30.html boards have been ordered.
Fans are for what?
Can't you be satisfied with just blinking some LEDs.
I think first you need to research the state of the art on various R/C airframes
had to carry about 450kg of gear
or head for the edge of space
charge a battery during the day and then discharge it over night into a heater to keep the hot air up,
This sounding any more realistic?