Hi, I would like to offer you my expertise in embedded firmware and software development, hardware and PCB design.
On numerous I've done similar project contaning gsm modules, sensor interface and bootloader.
I can offer you both firmware development and hardware design, and hardware prototype assembly if needed.
Although from your description I got an idea that you need FW only? anyhow I'm fine with both.
Yeah, cortex M3 should be good for this, and this would also be my suggestion. Although any of Cortex M3 is fine by me, I personally prefer STM32.
For the TCP/IP part we can discuss because there are two approcahes, we could configure modem to PPP mode and use our TCP/IP stack like lwIP, or we can use TCP/IP stack implemented in modem over AT commands. There are + and - in both approches. Also I would definitely use RTOS for this.
Bootloader development is also something I have a lot of experience and isn't a problem.
Feel free to contact me over chat.
Thanks,
Vladimir