An additional power supply is not required and you do need the correct expander card.
I would still use Low Current and/or SCB mode on the bell myself to reduce the current drawn by the sounder.
The expander card comes with instructions, it requires an EOL resistor in the sounder but other than that it's simple.
Yes, the GSM sends the same type of signals as PSTN, text message have zone information which is good. You have to set them up correctly with test signal enabled otherwise the SIM will expire.
This is a one-way systems so not the most secure, would have to be a low risk site IMO.