HA-Cluster and Proxmox?

I would like to find out more about HA Clusters and MongoDB. I would like to use 3 clients with Proxmox and run 3 Debian and 3 MongoDB Instances.

How can I best calculate the Hardware, how much performance do I need, for how many read an write accesses? The DB receives approx. 100.000 new entries per mpnth

Is there a hardware recommendation from MongoDB ?

And how would one proceed step by step ? Thank you very much.