Pretty cool/interesting. For the most part, I've stuck with marked and try to stick as closely to gfm as reasonable. I have on occasion added additional emoji support, but that's all I've found that I needed.
Recently added a layer to present a medium-style editor over the top, and that has been a huge set of frustrations in order to filter for nefarious content (xss).
Part of the exercise is to practice writing clearly. This is a very important skill in a number of fields, in particular medicine. When legal, medical or engineering notes are hard to read, especially potentially a while after written there is the opportunity for bad things to happen or the inability to find context later on.
While I appreciate the effort and think the project is cool, I find that the purpose of side stepping part of the exercise/assignment isn't such a great idea.
When generating your droplet, just use your public key instead of a passphrase here, it will allow you to skip several steps.
I would also suggest using dokku and add the letsencrypt plugin once you have your app in place. Deployment becomes as simple as a Dockerfile and a `git push deploy` from your repository. It takes a tiny bit of work, and really less than the article's approach. Note: make sure to run the command to setup the letsencrypt cron job. And you can set the letsencrypt contact email as a global config entry.
You may want to deploy dokku in your own ubuntu container just so you're more prepared for updates, that said it's pretty straight forward and will allow you to host several applications.
One warning is that if you're using the $5 droplet, you should setup vram. I tend to go for the $20 droplet, but I can host a dozen small sites without issue and get easy https to boot.
http://dokku.viewdocs.io/dokku/