Hi Jim,
I think you should be fine. I'm using five rescued
HP ePCs all with PIII @800MHz, 128MB RAM and a range of HDs, including one one with 80GB. All connected via a HP ProCurve 1G switch. I have one RPi2 connected to the cluster and it, too, works fine with them. Still having struggles with auth, etc.
For what it's worth, I have an ePC booting off a compact flash card (via an IDE adaptor) - that one is rather slow. I haven't benchmarked this cluster against anything, but my impression is that it's Ok, but a single contemporary PC seems faster (SSD, SATA3, etc)
Well that's one dodgy data point for you!