Not to nitpick, but the Contax prism finder isn't strictly an auto exposure attachment, as Kyocera built the meter into the body itself. Which is also why metering is so easy with the flip-up waist-level finder. The prism finder adds center-weighted metering to the body's spot metering.
You may already know this, but the C645's body finish can get degraded over time. It's just a coating, the rating of an item may be lower because it looks "ugly" and as long as it wasn't abused I wouldn't worry about appearances.
Contax 645 AE Prism Finder MF-1
The prism finder adds center-weighted metering to the body's spot metering.