I can not say I know much about edge connectors being used in sockets but I do have a lot of boards manufactured so maybe I can add a little to the conversation. From the place I get my boards made you get the choice of HASL, HASL (lead free), ENIG (immersion gold), Bare copper (by request) and Hard Plated Gold (for Fingers by request). Of those obviously the Hard Plated Gold is ideal but I think the ENIG would be fine for a board only being inserted once or twice. Even the HASL ones would be fine as they do not oxidize like bare copper.