I think you are overbuying on the CPUs. You really only need a single socket. I'd recommend that you buy storage up front. Growing your HA configuration will require downtime so don't go into production unless you know you have the storage you need at least until your next major maintenance window.
I'd try to get 10GB for replication traffic. The speed of your replication will limit your maximum throughput. In other words, you can't expect to write at the speed of light if you can only replicate a the speed of sound. The replication is synchronous. The FS isn't notified that the write is completed until it's done so on both nodes.