The more information you have at hand the less risky a N2 integration will be and the more successful the project will be. In preparation for a N2 integration we always recommend that you:
Use HVAC Pro and create a Network Map report with it. This will tell you the hardware type, Firmware versions, and application loaded into each N2 device.
If you see any Generic IO / Point Multiplexor applications this is a red flag. That means that the logic driving those points is back in an NCM, or other supervisory controller. You can still do a very successful integration. You just need to do more planning.
Make sure that each N2 bus segment is within electrical specifications. You can use the ComBus Quick Tester or a DVM. You will be driving the N2 bus harder than it has ever been driven before so any weakness will come out.
Review as built drawings and any other documentation that is available for the Metasys installation. I've taken the approach of shortening the N2 bus runs wherever we can to minimize the chance of interference and improve performance.
I also am trying to get rid of the repeaters when we can. If they start failing in the future they will cause you headaches that you don't need.
If you don't have in-house Metasys expertise you should seriously consider bringing a consultant or subcontractor on board who has this experience, even if it is only on an on-call basis. The S4 Open appliances very effectively provide the communications gateway functionality that publishes points on legacy N2 devices to open interfaces like OPC and BACnet. However, they cannot diagnose a controller improperly wired to a mechanical system, or an improperly programmed N2 device, they cannot tell you when a controller needs recommissioned, etc. Don't guess about the condition of the original installation. If you are not comfortable working with the Metasys N2 devices yourself make sure you line up the expertise to support your efforts. A little bit of insurance goes a long way.