Start with your best guess low end package and upgrade/add more resources as you need to. You can’t downgrade though.
We’re on OVH. Small community, 2.5k/reads a day. But I will say that even the lowest spec OVH server barely broke a sweat in terms of CPU. 3-5%. RAM is always maxed but that’s the nature of the OS.
One thing to note, Discourse likes fast CPUs over number or cores.
The main problem I had was insufficient disk space, that was because I stored too many backups on there. Rookie error 