It sounds like you already know about the Boot Block date that you can see in BIOS easily, and know the date to look for. If the workstation is already running the Xeon E56xx series processor(s) then it is the newer better version of the motherboard. I have used that approach in the past for buying used Z600 workstations for a donation project locally, and that has not failed me. Also, sometimes the eBay seller will show a screen shot picture of that first page in BIOS that includes the Boot Block date. That is a certain way to go.
If you buy one of the very slowest version 2 ones you can find it will be cheaper, as you are going to upgrade the processor(s) with a used faster one off eBay. A key piece of info.... buy a dog-slow one with 2 processors. Getting an extra heatsink/fan this way is wise. Those are not cheap to buy as a single item, the regular ones cover all processors up to and including 95W max TDP, and those will work with the two processors I list just below.
Processors..... X5675 versus X5672. The first is the fastest official version 2 hexacore. The second is the fastest official version 2 quad core. Clock speed of the second is faster than the significantly more expensive first. I have one of each here, both with two processors.
If you want to spend a lot more money you can supposedly run two of the X5690s in a version 2 Z600..... but you'll need two of the "Performance" heatsink/fan setups..... and that is quite an added cost also. The idea was to get the fastest possible at the least cost, and for that I chose the X5672 (two per Z600). Rocket ship, plus it gets a HP workstation into their hands. I chose 8 cores running faster over 12 running slower.