These are quite nice PABXs, and yours might even be ADSL capable.. As you're probably already aware, the handsets are "dedicated" to the system (won't work on BT lines or other makes/models of PABX).. I have both a 100, and a 200, although my 200 is badged as Avaya (who took over SDX in the late 90's).
As a system goes, it's now considered 'old hat' but is still a very capable system.. Be aware that you cannot reposition any of the plug in cassettes in the cabinet, unless you have the system setup software, password, and a suitable modem cable.
If you can list the cassette models, I might still have a list of types and function.. The model numbers are on the small label at the top of the front face of each cassette, and will read something like "DSLC-32" - "ALOG-8" - "E-PRI-30" - "AL-0-24" - "DPNSS 30" and "CPU200" etc.
You should have one "CPUxxx" cassette and at least one "DSLC" and one "ALOG" or "AL" type cassettes.
The setup software and installation manuals should still be available on the Avaya website (certainly were four years ago).. System programming can be done via "Kermit" or Windows "Hyperterminal".. You will however need the command codes and the password (many system were simply left as 0000).
If your not too bothered about changing any of the system configurations, you can simply plug it in and play.. The important thing to remember is that you cannot remove or change the position of any of the cassettes unless you have the software to reset the system - So, if you haven't already done so, don't rearrange the cassettes!
It's a nice system that could be expanded to carry upto around 1000 extensions (with extension cabinets and suitable cassettes).