hey grace,
some time ago i was working on a similar project. The tool i used back then was called "MWconn"... it allows u to automatically send sms.
www.mwconn.net/
All u need is a GSM Modem (and SIM card ofc).
When i used it i tried different old Handy models but most of them either had no GSM Modem, or had compatibillity problems. But in the end it worked perfectly with just using an UMTS stick.
to send SMS with the tool u just need a batchscript to generate a .txt file and move it to some MWconn directory. And all the files in the specific directory will be send automatically.
Hope this is what u were looking for.