Good luck, Schatzi... "non-expensive" is the hard part. I can personally recommend Extensis Portfolio from my day-job experience, but that will cost you... although I have no idea if you can find used or outdated copies out there for less. As software goes, it will do everything you need.
I've heard good things about Adobe PhotoShop Album... relatively cheap (~ $50 US) and some comparisons to Google Picasa (~ free US).
I'd be interested with what you come up with, as we're always looking for something like this that isn't too expensive, and that you don't get charged extra for each additional client you add.
Good luck .