1- you possibly first talk about Linux distributions "I wouldn't call it systems" and each one has its own pros and cons. However, they are all good to go with, and the most popular one is Ubuntu, but I go with Debian as I favor it on others.
All of them are easy to setup and doesn't take time even to learn how. The time consumption to know everything depends of course on whether you want to learn deeply or just in simple way.
I hope that answers both your 1st and 2nd questions.
2- to know the answer to your 3rd question, read this tutorial:
3- you can load your project through SFTP, not FTP.
4- It is always best to use SFTP since it is secure and safe to use, and avoid a lot of risks. also SFTP is way much secure than regular FTP. you can use filezilla to do so.
read this tut:
5- There are a lot of tutorials (tuts) you can find at "DO" DigitalOcean community here:
This article explains how to manage your DNS records using the DigitalOcean Control Panel and can help you decide if DigitalOcean's DNS management tools are right for your situation.