I think you will probably interfacing with sensor and a gsm module which usually interface using uart and audio input/output. For the sensor, you might need to see if the sensor output is dc or ac voltage. If ac, you might consider an adc if you need accurate reading from the sensor.
For the fpga part, the most direct solution will be a soft processor like niosII or microblaze which provided by vendor and hook them up with controller such as uart and etc. it will just work like an microprocessor like arm, 8051, and pic. So, if you system only interface with few sensor, a low cost fpga like cyclone or spartan series fpga will do the job. Just make sure they have enough GPIO for your entire system. For starting, just use their development kits that have uart, audio in/out, enough GPIO (adc maybe optional) and enough amount of memory for storage will do. Don't forget to check out their soft processor reference design to help you started.