use ab or similar to do an actual benchmark/stress test - although I know for a fact it'd probably run on the smallest droplet if you swapped out Apache for nginx or litespeed or something along those lines
Ideally you'd want PHP running as an FPM process with a decent pool configuration and MariaDB over MySQL .. because of course. If you can keep email off the same server then do so, making use of something like msmtp for relays or exim if you need just a sendmail alt.
That said as with any community or website, as visitors grow and the user base grows as do the costs but for most people a base droplet would be fine - also take all my suggestions as is .. Digital Ocean, linode a fair few others have some good guides on stress testing and picking the ideal server setup, I'm a big fan of start small, monitor.. something like what New Relic is excellent for monitoring singular setups or masses of them (no affiliation or relation by the way)
1GB 1CPU with plesk & Redis
This barely meets Plesk requirements on its own, unless you plan to host a ton of clients right away, alternative options exist (Hestia, Vesta) which are all a little lighter (on features, cost, resource cost)
