Rocket.Chat recommends at least 1GB of RAM and from my experience, 1GB should be the minimum that you deploy, so that’d be the $10 Droplet.
As for installation, you can actually install the latest version with very little effort using Ubuntu. You’d deploy a Ubuntu 16.04 64bit Droplet, run a few commands from the CLI, and you’ll be up and running in no time.
For example, on a fresh Droplet (you can copy and paste this to the CLI):
sudo apt-get update \
&& sudo apt-get -y upgrade \
&& sudo snap install rocketchat-server
And then follow any on-screen prompts.