I run Ditto and Postgres on separate servers, because I point multiple Ditto instances to the same Postgres. Ditto itself should have 2 cores and maybe 2-4GB RAM. Postgres​ should probably have at least 4 cores. On the same machine you could probably do 4 cores, 4GB for Ditto+Postgres to start with.

You should absolutely bring your Discord people there, but I don't think it will replace it because there's no private group chat. Check 0xChat for that.