If you speak spanish, the telcel blackberry site will answer most of your questions:
Micrositio BlackBerry de Telcel
Your Tour will work perfectly on the Mexican 1900 MHz GSM/GPRS/EDGE networks.
Each of the four telcel prepaid "amigo blackberry" plans includes a data "bundle".
You can tether your blackberry to a computer and use it as a modem for internet access, if you wish.
Sorry, I have no experience of the google voice app.
Once your amigo blackberry plan has expired, your blackberry will remain locked to the telcel network - until you return to Mexico again
But you can unlock it by using one of the many blackberry unlocking services on the internet, if you really must
