I didn't know Digital Ocean before, but it seems like they are more focused on CPU than on disk space. This is bad, since you will need to store some data on the server, if you send any images at all. We have a matrix-server running for 4 people since maybe 2019 and it's at 60gb disk usage right now. So in that regard, 10 memers will need at least a 100gb so 24$ a month, but you can get a lot cheaper than that, using some other server hosters, which are more focused on giving you a lot of space instead.

Specs: The server we use has 4 cores (barely uses 1) and 8 gigs of RAM, 2.5 of which are currently used and 120gb of disk space, that will probably suffice for another 2-3 years.