I agree with you, probably the best place to start is the original trilogy. Prelude to foundation and forward to foundation are nice, but I found them less impactful than the foundation/foundation and empire/second foundation. Since it ties to the foundation lore as well, I would suggest the robot series as well, it has some of my favourite books (the naked sun to name the best IMO!).

End of eternity is nice and could be seen as a part of the robot/foundation saga, but I would read it as it is and then, if you want, tie it to the overarching story.