Yes, of course, you can go through the below answers to your questions:
1. Following are some of the Open source tool for server monitoring systems:
1. ZABBIX
2. Nagios
3. SolarWinds
4. ManageEngine OpManager
5. Hyperic HQ
6. Sciencelogic EM
7. Spiceworks
8. OpenNMS
9. GFI Network Server Monitor
10. Paessler
2. CSF is a software firewall which is extensively used to on Linux servers to safeguard it from dangerous attacks like brute force, DOS, invalid login attempts, SMTP errors etc. Generally, Web hosting provider across the globe rely on CSF firewall. It works with iptables in preventing your server.
CSF is more compatible with Linux distros which is efficient to manage and install. You can easily enable setup/block ports, allow/block/ignore the IP access & perform as many tweaks with CSF firewall.
3. The vital importance of maintaining and updating your data cannot be ignored. Also, endure that your database is always secured and protected against SQL injection. It is even more essential when you collect sensitive information about clients.
Moreover. you must look for:
1. reducing privileges of database users
2. eliminate unwanted data and
3. deflect the areas of interaction between client and the database when not needed
Hope this helps