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Cluster Architecture Advice
(alien.top)
They have been having some nice deals on 2.5G unmanaged switches on Amazon for less than 90 bucks.
Start with the cheapest suitable switch and go from there.
I don’t know your aim but if you want performance: you have two PCIe slots so you could put a 10GbE card in each Optiplex then switch with a Mikrotik CRS305.
Check on the deals for the switch, it might be some especially on the upcoming Black Friday and Cyber Monday.
Also, you would need shared storage for the cluster. For the common OS, the best option for 2-3 nodes is starwinds vsan, it also doesn't require witness node for 2-node configuration, so you might save some bucks on it. It has minimal hardware requirements so, just put the drives and you are in.