I found this node really interesting: Encrypting Credit card numbers. If I understood correctly, you should not store your valuable data on the same machine connected to the outside world. In that node you will find many interesting and useful discussions about many related topics.

Probably you will need also some kind of versioning of your data. For the non-repudiation part, you may find interesting this link: Non-Repudiation in the Digital Environment

HTH, Valerio