You will most definitely need more security than just the default server configuration. This is why people use PaaS so they don’t have to deal with this. Think of PaaS as outsourcing your server administration.

Scaling on most PaaS ist easier than on a VPS. Like @Qqwy said, if you’re doing this as a hobby project, you might consider setting everything up yourself a useful learning experience. But if you’re not yet experienced and plan to handle real user data, you can’t just start an Ubuntu VPS and be done with it.

If you are doing this as a hobby-project, however, I would like to say: do use a Virtual Private Server. There’s no better way to learn something than doing it yourself, and there is a plethora of tutorials out there.

Do you have experience administering a Linux server? If not, you might not want to use a VPS.

Instead, take a look at PaaS services; You can run Elixir on CloudFoundry or other systems via Docker, even Google App Engine now supports Elixir or you could go with a specialized service like Gigalixir.