When a chessengine or user enters a SAN move, do I need to know what
moves he is capable at that moment of doing? Or is it enough to know what
pieces are where on the board?

You should be able to get the exact move given just the pieces on the
board and a correctly formatted SAN move, that includes all proper
disambiguation such as Ngf3 or Rhd1.

Ah ok, really glad to hear that. Thanks!