I think you should plan on expert installation of the 900 MHz, and have the expert choose the antenna and other specifics. 2.4 GHz (802.11 b/g) can also work over such distances if done right.

Why Skype? Sending out SMS messages via email is far simpler on the infrastructure, works in low bandwidth scenarios and is store & forward: Skype probably has some unknown bandwidth floor requirement the radio operator has to meet (and I think some HAM schemes are quite slow, 9600 bps nominal or less).