Z400, Z420, Z600, and probably Z440, Z620, Z640, Z800, Z820 should all do just fine, those 3 at the start i have tested over 20 by now. Also HP 8300 SFF are pretty good, i have maybe 6 or 7 in daily use on machines.
Dell Optiplex series are also good, 7020, 9020 and 980, these i have in daily use, several of them for several years, but they do tend to heat much more than HP ones.
