I can't give you any specs, but I can confidently tell you why: each problem will be different enough, and sometimes may have plugins that have certain performance characteristics with certain hardware, that you will want to make what you plan to run on it known up front, just to be able to get useful advice, for the machines. Some may do well with AMD (yes, even today), some Hyperthreading (i5 v. i7), some may do better with many cores per box v. many boxes (desktop v. server sockets), etc..
Aside from that, get an unmanaged Netgear or Trendnet business GbE switch, with enough ports. Not one of those plastic jobs, but something like an GS108 or GS116; and a decent surge protector or UPS (UPS adds cost, but allows for nice shut downs, if you properly configure and test it). Switches are cheap enough that you can have a spare, in case it blows, or just get a Netgear that's on the shelf at Staples or BB, to easily get an exact replacement if needed, same-day.