moved to Building your own PC, just in case.
A system based on a decent Core i5 would leave a little more room in your budget to get while allowing you a wider range of hardware choices. You would have more headroom to go for 32 GB of RAM (for 6 concurrent virtual systems with 3.5 GB apiece, plus the memory for the host OS, 24 GB would be fine, I'd think), and a decent consumer motherboard could be had for less than the cost of an i5.
The downside to going with the i5 is you don't get HyperThreading (generally), so the 4 cores you get are the 4 cores you get. But you can't step up to a Xeon-based system without incurring more costs in one area or another, so staying with a build based on consumer parts seems ideal.
As for the power supply, Seasonic is a reputable brand. Depending on the amount of storage you'd have, 400W or 500W from a Seasonic (or Seasonic-based) power supply would work nicely. I'm not sure how other brands' power supplies are at this point in time, though I think most reputable brands are pretty solid regardless.