I think it really depends.
Which Summicron 50/2 is that? When you look up KEH, there are quite a bit of price differences between models.
Also I think you need to inspect the M2 further. It might look excellent shape, but unless the owner can show you proof of CLA, you might want to count in the cost of CLA anyway. Bring a pen light and see RF and shutter curtains for potential issues.
I had one M2 body which looked excellent, but the mount was very stiff and it turned out there was a distortion on the frame I had to return the camera.