I would go for such a card, just for IDE. I would love to have WB 1.3 set up on my A500.
RAM is not an issue. Could be as low (cheap) as possible. I don't know what is the lowest possible solution Jens has around 16, 32MB...?
Better CPU ? Not a necessity. But if it could be done with a cheap 14 MHz 020, fine.
And Zetr0's idea about getting a CF socket tied to IDE is brilliant. Would solve a bunch of possible problems.