Although an NM could certainly help a 1900 rated player, I think that a coach should usually be more than 300 points higher than the student because 1900 players sometimes do beat NMs, especially if they are currently improving. I took a few lessons from IM Stupavsky when I was 1850-1950, and I felt it really helped me, but I honestly don't know that an NM would be able to help as much, especially because I frequently played them and would often draw and occasionally beat them. As general rule I'd say try to get a coach at least 400 points higher than you, although of course there are exceptions, especially when you're over 2000.
As for the repertoire question, I'd say ask the coach. Most coaches probably wouldn't mind because most of them would know the ideas of most openings.
When selecting a chess coach, how important is it that the coach play similar openings to what I currently play. For example, should I exclude a coach because I am a 1. d4 player and the coach in question plays 1. e4. Or I play 1. ...e5 as black but the coach is a lifelong 1. ...e6 player.
How much should I let title (GM,IM, etc.) influence my decision? If I'm a 1900 USCF for example, is selecting a NM coach going to give me the improvement I'm after?