What we did:
-Obtain a used HP Proliant DL380 G6 (2x Xeon E5540 | 48GB RAM) or something alike - any brand is fine.
-Get any kind of graphics card. We used a GT 710 2GB we had lying around (~EUR 35 from Amazon, new).
Benefits in comparison to a regular tower PC:
-Multiple of these fit into a regular 42U 19" server cabinet (total of 18 machines with decent ventilation, patch panel as well as some additional hardware)
-relatively cheap as obtained used & refurbished
-easy exchange of HW in case of fault or defect.
Software we used in addition to HB:
-HBRelog/aRelog or whichever relogger you like best
-DirectX Rendering disabler (pick whichever one suits you best). Imho the only reason why the whole project worked with the GT 710.
-While optional, we found this to come in handy: RAM Cleaner - again whichever you prefer. Just make sure the tool is capable of running headlessly.
-BuddyMon (will be outdated/down as of now). But be sure to set up some monitoring interface.
-Remote desktop of some kind (MS RD/VNC/Teamviewer/etc.)
We ended up running ~60 bots on this setup with no problem (per DL380 that is). Still I'd advise to run 40 per machine, for some reason we had significantly less WoW crashes & especially less problems with the relogger running that amount.
Important to mention is that we did not have to pay any electricity costs.
But coming back to your original question:
-Depending on your location & the money you plan to spend: I'd get a used PC, something like an AMD FX 6-core, 16 gigs of RAM, any type of graphics card.
-Use the software setup described above.
Kind regards,
p4mdude