Are you trying to protect your system from unauthorized access or data loss.  Security comes in several different forms.  If your strictly worried about unauthorized access us standalone PC that is not connected to the internet.  You could have several networked together as long as they are not connected to the internet.  Personally, I'd go for a server hosted, in-house application running on a enterprise grade server.  This server needs to have a robust RAID setup for data storage (redundancy), multiple power supplies, an automated backup plan, and also needs to be running behind a commercial grade firewall.  Access to the server needs to be controlled and monitored by a server running Windows Active Directory or something comparable.

I'm not a fan of cloud based services UNLESS you have some sort of relationship with the cloud provider and can access it via a secure connection.  The bank I worked at and still do contract work has a service provider that handles nearly all their banking software.  All customer data is stored and managed in their data centers and the bank connects to them.  The actual customer data is not at the bank itself.  All this company does is banking.

In my own house I used to use an online backup service.  They did a good job but I decided to put a server in my storm shelter and handle it myself.  I know where the data is now.