...she finds out she has inherited a large sum which she doesn't want from Lex. She gives it to Superman and this I think is what starts the Superman Foundation.
You know, I'm pretty sure this isn't the fic you're looking for but check it out anyway. If you haven't read it yet you're in for a treat.
It's
"Master of Disguise" by BJ.
And then of course there's the wonderful
"Interest Bearing" by JDG. Both are on the Archive.
Were these the ones you were looking for?