My project is essentially a box that will have LEDs/buzzers powered by relays and the GPIOs, doing it as a wired setup will be one option, but I'd also like to include the Invonics wireless system as well. This would be easy for me to purchase the Inovonics receives with built in relays and programming, but I want to avoid that cost. The serial only receiver EN4200 is just like the all in one receivers they offer, but it only outputs via serial what it sees from the Inovonics transmitters, whether that be alarm, battery low, supervision fail, tamper, etc. The EN4200 from what I can tell is designed to be integrated with other controllers that can read the serial format, thus eliminating the need for the full controller. I would imagine it wouldn't be much of an issue for the Arduino to read the serial data and trigger GPIOs from it, but it also needs to be able to enroll and delete the transmitters and communicate with them for supervision.
So again, the EN4200 will be receiving the "Echostream" data from the transmitter buttons and translating it to serial data, where the Arduino would then trigger GPIOs depending on that status.