I can appreciate your need to want to think this through, but like the other poster inferred, its not a big deal. As long as your cluster is sized for at least N+1 from a host perspective, then as they age, replace them as needed. Rolling hardware upgrades happen all the time, and they VMs none the wiser. Their CAN be some benefit to keeping the hardware as similar as possible (chipset features, and calculating HA N+1 numbers when you have similar amounts of RAM, and compute)
I recently had a situation where I replaced 4 of my 7 nodes from old harpertown based Intel processors to Sandybridge processors. Double the amount of physical cores (8 compared to 16), quadrupal the amount of logical cores (8 compared to 32), and 6 times the mount of RAM. That progress easily absorbed the internal demands. It was such a significant change that it made a legitimate argument for not using up valuable vSphere licenses on older physical hosts. It's just not worth it.