1. Is it even possible to host them all on one droplet in the first place?
   It should be but you will need to set up a reverse proxy (proxy_pass) so the Ghost site is served via nginx.

2. Should I set up with LEMP first? Then install WordPress + Ghost? (If what I read is right, too many visitors and an Apache server don’t usually play nice)

I would recommend setting up Ghost with nginx in front first, then add additional server blocks for your WordPress sites.

1. If there are multiple sites hosted on a droplet, how should I point the DNS servers to the right site?
   Your nginx configuration will have a server_name field for each site. You will simply need to point all domains to the droplet’s IP address and nginx will take care of the rest.

2. Would the installation of a GUI [manager] like Webmin help me out with setup/config?
   For this type of configuration it likely would not. For basic LAMP sites this can be useful but it also puts a layer between you and your actual configuration so tracking down errors or issues could be more difficult.

3. As there’s an eCommerce site, security is paramount. What should I look out for when setting up. (I feel like this is a broad question, from actual settings to philosophical practice, throw a couple of updated guides in my way and I would be set)
   First you should absolutely implement https. LetsEncrypt will let you set up an SSL certificate for free.
   This guide wil walk you through some initial steps to secure a new droplet
   This one includes some nginx specific steps you can take for security.

4. Is the DO 1GB sufficient for my needs?
   Will it run these sites, absolutely. Whether it is enough to meet your needs depends on how you optimize and how much traffic you receive. We see this question a lot and the answer is almost always, you’ll need to do some testing on your own with your actual configuration and scripts to know for sure.

5. Which Linux distro should I go for? (Or they really are the same and I’ll just follow whichever the guide presents)
   For the most part it is a matter of preference. In my own opinion I would recommend Ubuntu as it has the most active community and in our own library of tutorials it is the best documented.