[Gdeaver]

A Freescale pressure sensor with the proper ADC can easily give altitude with in 5 ft. To measure air speed use a tube perpendicular to the airflow and measure the pressure drop within the tube. I forget the name for this device it begins with a p. The MPX4115a is the most used sensor for these apps.
There are GPS units about the size of a quarter that interface with a PIC by serial. There are allot of sources on the web on how to decode the packet. A 3 axis digital compass would also be a nice addition but they are expensive. You should be looking on the RC airplane forums. You'll find allot more specific how to's.