It very much depends on the size of your instance- small instances run perfectly fine on a Pi, but you may want to at least get 2-4 cores and 4+gb ram to start, plan to grow over time (and/or scale horizontally with some extra UI nodes or whatever you need). Storage can either be nothing- if you disable local image uploads- or grow slowly with your userbase. Avatars, user uploads and cached thumbnails will all pile up over time.