If you want to understand distributed-memory parallel performance you're probably better off with a simulator, like SimGrid. I don't know what bog-standard hardware you'd need to get a typical correct balance between floating point performance, memory, filesystem i/o, and general interconnect performance otherwise. No toy system is going to teach you about running a real HPC system either -- you really don't want the fastest system if it's going to fall over every few hours or basically fall apart after a year.