John,
The prices will drop over time on eBay. They have already for the Z600 processors. I looked last evening and a used version 2 motherboard off eBay is about 150.00 USD. Note that each part for HP generally has two part numbers, a Spares P/N and an Assembly P/N. Some of the ads give both, and then you can search for both. The part numbers in the "Z420 Spart Parts" search only gives you one of the two.
If you see one with a cover over the processor socket pins that is a good sign...... those pins are very delicate. Most used motherboards will not have that because those covers generally get thrown out. That may indicate it actually is a new unused motherboard.... Some shops break these workstations down because the value of the parts sold individually adds up to more than they could get by selling it as a functioning unit.
BIOS update..... google Z420 drivers, and the HP result at the top will get you to a list of OS.... it does not matter what you choose because they all use the same BIOS upgrade. I just checked.... a new one came out 11/3/16, version 3.91, SP78208. Download that SP, and you can upgrade BIOS using one of the below methods (safer v.s. easier):
1. Safer.... Upgrade BIOS from within BIOS. Run the SP, cancel out after it has launched, go into the root level of your C drive, find the HP folder there SWSetup, go in there and find the SP, go in that folder into the DOS folder and pull out the .bin file from there. You may need to turn on view of known extension types in Folder Options control panel, View tab. Then have that on an otherwise blank thumb drive plugged into you black front USB2 port, and cold boot into BIOS and under the first tab of BIOS you can see the option to update system ROM or some such statement. Click on that and point to the USB drive. That is how I do it, for max safety.
2. Easier.... launch the SP and choose to update BIOS from within your OS. I get the SP onto the desktop, and then do a cold boot from full shutdown and launch that SP after the boot has finished fully, with no other programs going. It will take some time, but works. If your BIOS is way old there is an intermediate BIOS update you need to do before the newest ones will work. You don't need to go through this if your BIOS is new enough. You don't need to install each individual version that has come out over the years sequentially because the newest one includes all the needed fixes that the older ones had.
Scott