I think the main interest was the dirt-for-blackmail, that someone paid much for. Where did that couple's wealth come from?

I think Ghislaine was groomed from a young age to spy. I.e. she knew how to do it, and that was important, seeing as daddy died in 1991. Someone had to carry on, in the 'business'. One must carry on the lifestyle that as one has become accustomed to. So it was a match made in heaven...