This is an old thread, but since I just wasted 5 hours on tracking this down there are probably others out there.

As someone pointed out, any power considerations are only necessary for add ons not including the display. The document that I mention below details all the aspects of these models, it's just spread around the document. I'm currently using NTZK sets that have handsfree and displays without any additional power. I was confused by the sparse product descriptions when I looked some new sets, thinking that I needed to care which version I bought.

Reference document:
Telephone and Attendant Console Installation
Doc 553-3001-215
v12 Jan 2002

From Doc:

Note: There are three distinct versions of M2000 Series Meridian Digital Telephones – all three are supported. The versions can be clearly distinguished by the first four letters in the upper left-hand corner of the model identification label on the bottom of the telephone.

The three versions are as follows:

• the“NTZK”models,
• the “NT2K” models with date code prior to April 24, 1998
• the third version includes both the “NT9K” models and the “NT2K” models with date code of April 24, 1998 and later. In addition, the two jacks face in the same direction on “NT2K” and
“NT9K” telephones, and in opposite directions on “NTZK” telephones. When appropriate, differences between the models are noted in this
document.