SDH,
Yes, it seemed to me that the similarity of the layout of the z210 and z420 motherboards, that the chasses ae identical, that the output specs of the z210 and z420 400W were identical, that the connectors and cable lengths would be compatible, that I simply swapped the 400W for a 600W, and that HP is likely to produce the fewest possible number of 400W PSU's to be used in the same chassis. made it more likely to work than a third party PSU- which we know has different mounting points and main connector. Still, as I mentioned in the post, this should be researched in more detail.
I like the "new" budget z420 project. Z620_2 was assembled in the same way: new chassis/ power supply ($180) used motherboard, RAM (64GB DDR3-1866 ECC reg), liquid cooler, drives (Z Turbo M.2 + Intel 730 + HGST 7K6000 4TB), and GPU (Quadro P2000) transferred from z420_2, new CPU (E5-1680 v2). I think the net cost of the change after selling z420_2 was about $500. That low net cost has been spoiled a bit as I've ordered a GTX 1070 Ti.
What are the components going into the "new" z420? To keep costs very low, how about an E5-1607 v2 and Quadro FX 580? I heard of a drawer where those two components may be found for one astoundingly low price. And those will run on the 400W PSU. The Quadro FX 580 - I've had four of them since I think 2004, is only a 512MB card, but, amazingly, was still on the Autodesk recommended list for AutoCad 2016. The 3D is sub-minimal these days, but the 2D is completely reasonable.
Actually, the 400W PSU is more useful today than it may seem as modern GPU's- the electron hogs of the system, are so efficient now:
https://www.studio1productions.com/Articles/NVidia-GPU-Chart.htm
Note that a GTX 960, GTX1060, and Quadro P600- good performers all, will run on a 400W. Probably, the best GPU for cost/performance is a used GTX 750 Ti - and those run on a 300W. Workstations are also more forgiving than we think- the PSU ratings must be conservative and have quite a bit of overhead. I tried a Quadro K4200 4GB- rated for a 500W power supply, in a Dell Precision 390 with 375W and it had then the highest 3D mark on Passmark for a 390. If cost is importannt, and given that some fast GPU's will run on 400W, it may be worth giving the 400W supply a go just to get started.
BambiBoomZ