Resources per user per post

I help manage a Listserv group with 2,000+ subscribers and close to 1,000 posts per month. Posts are often relatively long (maybe averaging 3,000 char), and sometimes with attachments. Can anyone give me a rough estimate of resources needed to support this scale with good performance? I have a test site working on DigitalOcean, and need to consider whether we can afford a move from Listserv (which we are getting for free) to a hosted Discource forum.

Thanks,

Its quite likely you could handle it on a 10$ a month instance, but I would probably opt for the 20$ instance to be safe.