If you don't have a lot of time it could be a good idea to use an SMS gateway, such as the Ozeki SMS Sending and SMS receiving Gateway available at http://www.ozekisms.com/sms_sending_software/index.php, because it does a lot of things that is very hard to implement: character conversion to unicode SMS messages, GSM 7 bit, segmentation and reassembly for multipart SMS messages with 8 bit reference numbering and 16 bit reference numbering, message delivered to network report, message delivered to handset report, message delivery failed report, etc. Not to talk about long term upgrade.
Tipp: In the trial version you can enable low level communication logging in the GSM modem connection, which gives you the ability to see what the software does. How it encodes the PDU-s. This is a good way to do some reverse engineering to see how to send SMS PDU's with AT+CMGS commands.
If you do a system now with a GSM modem, it might be possible that you will have to upgrade in the long term to higher capcity IP SMS connection, such as SMPP sms sending, UCP sms sending or CIMD2 sms sending. Remember, a GSM modem can only send 10 SMS meessages per minute, while an IP SMS connection will allow you to send 3000 or more SMS messages per minute. If you use an SMS Gateway, this upgrade can be done with a simple configuration.