If you ask me, this is common sense, and this is what many of my clients are doing now.
You can and should put all the security you can on your point-of-sale computer, but it's never going to make it 100%. Even the top security experts in the world cannot do that, but the simple truth is that almost all computer data breaches and ransomware attacks can be avoided by just being careful.
Do not allow employees or yourself to play on the computer, those computers are for work not fun. Make a policy about it. If you are prepared to let your employees surf on a computer on Facebook, celebrity gossip, download torrents, look at porn or do online gaming let them use their smart phone and your wifi so they are not connected to your network. If you are going to do emails on a computer attached to your system, be very careful where you go and what links you press. I know people that have a special accounts machine that is disconnected from their POS network. They transfer files if required only using USB sticks.
Avoid if possible online financial transactions that might reveal your passwords or personal information.
Frequently changing your passwords
Watch who is looking over your shoulder.
Do not let unauthorised people get into your system.
etc.
This is not rocket science, but it works.
If you want to enforce a policy a one good software, I recommend is to put on your network Norton Family available here.
The free version should be good enough for most people and tell all your staff, in fact put a notice so everyone can read that you have monitoring software installed, and that it allows you to monitor everyone on your network. It's not a 100%, but it should be able to do about 90% and it does monitor a lot. Together with your cameras it makes a powerful monitoring service.
Locking down a system can save you a lot of grief.
