I had an S2A outfit a few years ago. I had the S2A body and a second one that for some reason I'm drawing a blank on that was similar to the S2 but didn't have a removable film back and a top shutter speed of 1/500. I had lenses from 50mm to I think 240mm, along with the bellows and some other goodies.
I remember too that I hunted for a while for a dark slide. Mine didn't come with one, and I'm not sure if there's any work around to be able to remove the back without the dark slide. The slide is used to operate the catch that releases the back.
Despite replacing the mirror foam, my success rate with sharp photos handheld was very low. Sharpness was excellent-among the best 6x6 I've used with most lenses-on a tripod but I found it incredibly difficult to hand hold. There's a LOT going on in the body during an exposure-the mirror goes down, the silk bllinds roll out over the focus screen and mirror, and then the shutter opens. There's too much moving in too many different directions.
Also, I seem to recall that the 135mm Nikkor was not a great lens. IIRC, it was the same basic lens as the 135mm meant for use on Nikon S-mount rangefinders paired with a mirror box. It just happened to have a fairly large image circle and was pressed into service in this application. Center sharpness was okay, but the edges were weak even stopped down. My other lenses were a mix of Nikkor and Zenzanon, and the 135mm was the only one I remember noticing being bad.
I never had a working EC, but liked the split mirror concept with it and always thought I'd try to find a nice one. At the time, I also had an SQ-A, which I found an overall more useable camera. Somewhere or another in there I ended up with an ETRS kit too. Bronica made good stuff, but I never took to the ETRS because I didn't find the size/weight savings over the SQ worth it. At the end of the day, they all lost and got sold when I got my 500C.