First
At DigitalOcean should I choose the app: Discourse | DigitalOcean Marketplace 1-Click App
Or should I choose a vanilla ubuntu 18.04 and then install it using discourse/docs/INSTALL-cloud.md at main · discourse/discourse · GitHub
Second
The best way to move the content. Just backup and then import? Using
/admin/backups
Using discourse/docs/INSTALL-cloud.md at main · discourse/discourse · GitHub would be the way to go because that is the only officially supported method for now 
Yes, see Create, download, and restore a backup of your Discourse database for more details.
btw: when moving to digtitalocean I will change domain to a subdomain
studmed.dk —> forum.studmed.dk
anything I should be aware of? Except the dns record 
found this: Change the domain name or rename my Discourse?