Discourse is a pretty solid software and if you can install the forum by your own you can keep it running smooth. Just going to example.com/admin/upgrade every time you get an email or the notification in the Admin Dashboard and something like twice a year entering into the server and doing ./launcher app rebuild.
DigitalOcean keeps the servers pretty secure by default, if you want it to be even more secure you can get a firewall to block all the ports.
You can talk with any of the guys that offer an installation of Discourse here so they can get it set up for you (if you prefer) with a more technical initial setup.