I owned an S2a for decades and got some excellent images from it. Lenses are reasonable, in fact, downright cheap. And yes, the ground glass is dark. You can consider having Koh's camera install a more recent, brighter screen. As to the mirror slap, I always used the S2a on a tripod.

I think probably worse than the mirror slap when these focal plane cameras are used off a tripod is the 1/40th second flash sync. That means it takes that amount of time for the slit to pass the length of the 2 1/4" format. As I say, I always used it on a tripod for images that didn't include movement.

A warning on film backs, there are two types. The earlier ones had a black cloth that was pulled along with the darkslide when it was inserted and removed. The cloth is a light shield. Avoid these backs, because the cloth can get stuck half way when photographing and cover half the negative. The darkslides on later backs were light tight enough that the cloth wasn't needed. You can tell the earlier ones, if you see a photo of the underside. There's a black clip that fits over the end of the darkslide, and it can be seen even when the darkslide is fully inserted. One indicator is to get only those backs that are good for 220 and 120. Even those can have a black cloth, but not as many. Or if on EBay, you can ask.

There are still accessories and lenses available. I always searched "Bronica" in the search field at the top of EBay. You have to sift through all the later Bronica stuff.