Setup Server, Need Advice

I have a web app that serves about 10-50 people at one time. I’m paying for service at Linode currently, and I’m in the process of re-writing my app.

However, none of that is my concern. My server is running an ancient Debian OS, and instead of trying to upgrade it, instead I’m planning on spinning up a new server instance and just moving to that.

My question is, what UNIX or Linux distro should I start with? Does it matter much, are there advantages to some that I might not be aware of? (Debian, Ubuntu, CentOS, Fedora…) And is there an up-to-date guide somewhere that might guide me on things I should install or configure for this web server, for blocking malicious IPs, monitoring traffic, etc.?

Thanks in advance!