If you buy a new Nortel system, the manuals will come with. I would recommend CICS for a small office, 2-8 lines, less than 16 users, and a CallPilot 100. I would recommend MICS for an mid-sized office of 8-64 lines (using a T1/PRI) and about 16-96 users, and you could possibly add in DIDs and a CallPilot 150. Anything over that and I would say go to a Meridian1 PBX with DID and Meridian Mail.
The reason many people are going for Nortel these days is because of the end user's ease-of-use. They are also very robust systems, can be widely expanded (though there is a limit I recommend), and the equipment can be used for long periods of time before needing upgrades.
You might also check out the BCM line of systems. The BCM line uses just about all of the same equipment as a MICS, CICS, and the rest of the Norstar line.
John Panzer