Cannot NK Pro be used to store a single SSH server key too?
The NK Pro can only store three keys, or better one key and three subkeys for GPG. One of these keys you could use with the gpg agent to start ssh sessions. This is transparent to you as the gpg-agen is just configured inside gpg.conf. It will ensure that ssh use the token with pkcs11# lib
Why do I need HSM to store client key (you wrote about two HSMs), why not just only NK Pro for the client?
Once you have a NK HSM for your client, you could use it for multiple purpose. Of course NK will sell you another NK Pro for your ssh client as a user key.
Which hardware would you suggest to run OpenBSD free from Spectre, Meltdown and BLOBs?
LOL , this is a wide area for answering: what performance do you expect, what data will you connect etc. And of course this is not my area of expertise. There are also INTEL proz that don’t have these troubles. To my mind a lot of these errors will be found on Intel proz as everybody is focusing on them. But e.g. Cachelines are also used in other proz to speed up the accesstimes. So I have my doubts that ARM, AMD, PowerPC etc. have not a potential for leakage. It might be not found or - even worst - not published.
If you want to protect e.g. atomic rocket bombs, carry it as a suitecase with no network connection with you - then the proc for encrypting/decrypting is no longer relevant ( exept for performance ) 