Those error messages might be caused by something as simple as a low battery or it might be something more serious. If you are able to examine it before purchasing, try inserting a new battery on the MPU and see if you can clear the error messages. You do this by booting it up until it displays the error messages, then turn the reset key three times and follow the instructions on the display. It should tell you to press and hold the white test button on the front of the MPU for three seconds and then close the door. Hopefully this will clear the errors and it will run. If not, you will need to look a little further for the problem. Possibly the MPU.
Personally, I wouldn't buy a slot that doesn't work unless I got it for a pretty low price. If you don't mind working on it and like a little challenge, I'm sure you can get it running again with the help available on this forum.
Keep in mind the number one rule: Never connect or disconnect anything under power. It can do damage. Always power off.
Number two: If the MPU board has a green Varta battery, remove it. It is not necessary for home use. It will eventually leak and destroy the board. You can find instructions on its removal on this forum.
As far as the button blank, all of my S2000 machines are the same and have no button there. Maybe there's some specific game theme that uses an additional button? I really don't know.