Interface Asterisk with Comdial DSU II

I’m new to asterisk, and we are looking to possibly upgrade our telecom system at work.
I’m trying to determine if we can use asterisk with our existing system.

I understand the setup of a basic PBX setup with Asterisk, and I understand what would be needed to replace our entire system. What I dont understand is the use of Asterisk as a gateway to connect to legacy systems. This functionality is outlined on the website, but I cant find enough information on it.

Does anyone know if there is a way to interface with an older comdial DSU II system?
If so, is there any of the functionality in asterisk that would be lost in doing this?

I would like to use Asterisk as an automated attendant and voicemail system to interface with our existing system.
We want to create “ring groups” for each department, and have calls routed to departments based on the input of the caller durring the voice prompts. These ring groups would cause every phone in that group to ring at the same time, and the first user to pick up the phone gets the call.
We would also need typical intercom/paging (all call) functionality.
I havent gotten a budget for this yet, but it will probably be impossibly low, which is why I was looking at asterisk, but If I have to replace all of our telephones its probably going to become too expensive.

Any ideas?