Can't answer your M2 question, but curious to know which racks you are considering...
New 25U 4 Post Open Frame Network Data Server Rack 19" 1000MM Deep With L Rails
[URL='http://www.ebay.com/itm/201397107120?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT']StarTech.com 25U Adjustable Depth Open Frame 4 Post Server Rack w/ Casters[/URL]
[URL='http://www.ebay.com/itm/201397107120?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT'][/URL]
The StarTech one seems higher quality from the reviews I have read for both, and from what I have seen so far the StarTech brand accessories cost more.
The one from Raising Electronics comes with 3 rails already, so I wouldn't have to purchase anything but the rack with the 3 included rails right now.