The SIM808 is intented to be used as a GSM modem, to my best knowledge. This means that you can use it for creating an internet connection from your PI. There is a lot of documentation on how to setup the SIM808 (or other similar modems), to be used with the RPI, here is one:
https://www.cooking-hacks.com/documenta ... -tutorial/
And here is one for the 908 (which should be similar in interface):
http://projects-raspberry.com/geolocati ... pberry-pi/
You would have to write a small program program to do the texting. I would use a third-party sms gateway provider, like this:
But there are many others similar to this.
Good luck with your project, it sound like great fun