The NC/2 Is unmetered
I have a 500C/M and have 4 of the major finders they made for it
The classic waist level flip up style is the best if you intend on going places with the camera as its by far the most low profile and least obtrusive. Of course since you are viewing the screen directly everything is backwards. Id say this is the one that spends 90% of the time on the camera. The magnifier on the waist level finder is great for focus checking.
I personally like the chimney style finder for landscape work. When out on a bright day it does a far better job of blocking out light than the flip up finder especially when using something like a two stop red filter etc. Depending on how you mount the camera on the tripod looking down into it is also the same with this one as the waist level.
The prim finders come in two flavors, metered and unmetered. In general the prisms are nice as they rectify the image.
I have
a metered 45 degree prism (CDSPrisim) that I rarely use but is good when I need it. First and foremost its important to note that the 500C/M allows for a variety of screens to be installed. The metered prisms are POST-SCREEN and the various screens will allow different amounts of light through. To ensure proper operation you may need to adjust the meter compensation for the installed screen. I don't remember off hand which screen is intended to be the "zeroed" screen.
I also have the
later series 45 degree unmetered prism (PM5). This one is a bit brighter and clearer than the metered one (the mirrors are different inside think) but its a solid unit. Great for studio portrait work. This finder also adds a cold shoe to the camera.
My one complaint I have with the prisms is they don't have diopter correction so if you wear glasses (like I do) you either need to remove the rubber angled cup for comfortable operation or wear contacts (as I often do when using a prism). Keep in mind the prisms also lack the magnifier of the waist level finder so focus must be more carefully considered.
As for compatibility my understanding is that anything made for the V series will fit. You can also use some of the stuff that was made for the Kiev cameras (Russian 'Blad copies). The metering on a 500C/M will be post screen with any of the metered finders.
You can find a lot of info on the V series accessories here