I want to implement a SMS server which will be receiving the SMS messages sent from my devices all around the country. May be about 5 SMS per minute. I want to know if the RBP would be stable enough to do this job reliably if configured properly.
In your case I guess it's an embedded device you want(?). That is, the opposite of a desktop computer (headless, no screen nor keyboard). You may need to reduce the number of writes to the SD-card (since it has limited lifetime). Best way for that is with an operating system which mounts the filesystem read-only. Then you can be sure of that your application is the only one writing to the SD-card. It's also a good method for keeping the system in a pristine state. A simple reboot will revert everything to the original settings.