I'd be interested to know if you need a high-end CPU to go with the GPUs or whether the GPUs do all the work. I've got some old HP Proliant servers kicking about which can take half a dozen PCIe cards so it'd be nice to make use of them.
Electricity at the workshop is half the price of home too
Hmm the Proliants would only take two of those double-height x16 graphics cards. Is that worth it?