Thanks, that might be ideal but the secondhand market for "new" miners seems almost double the price of ordering from Bitmain directly — and I haven't been following these closely enough to be able to figure out which are old enough.
What about powering only some of the boards? Is that a feasible option or are there issues with that?
Or, are there any other suitable miners I've missed? I found the AvalonMiner 741 which might do, but it's out of stock and looks to be more equivalent to the S7 generation in efficiency.
Yes, you could run with only powering 1 or 2 of the three boards. That would work. As long as each board got its ~500w of power, you'd be good (plus the controller.)