1. You should update your BIOS to the latest. The BIOS is a primative OS that runs before your main OS, regardless of the main OS version. That is, BIOS does not know or care what is coming after it has done its job..... after boot it is the job of the main OS to be compatible with BIOS and the hardware present. That HP workstation works great with W10Pro64, and I even have W10Pro64 running very well on xw6400 and xw6600 workstations. I have recently posted here on using the Kingston Predator M.2 SSD as my boot drive on that workstation, which is even faster than SATA 3.0. If you use a PCIe interface read up on which PCIe slots in the Z600 are fastest..... regardless of whether you go SATA gen III or PCIe M.2 SSD you don't want to pick a slow one for your fast card.
2. Just because HP no longer supports a specific workstation with driver and OS updates does not mean it will not work. Many of the drivers HP provides are universal and work on older workstations. So, if you have a Broadcom and Realtek chipset on your motherboard you can go to get the installers from later workstations and they generally will be backwards compatible with the older hardware/chipsets too. That is, we do the testing ourselves at this stage. HP has to move on to the later generations.
3. Regarding that LSI card..... sometimes when you plug in a card into a W7/W8/W10 workstation there will be a proper driver in the hidden drivers store that the OS has in reserve, and it will recognize that and go get it and install it automatically. In other cases you need to do some hunting yourself. So, I'd start with that, and then go to LSI to see if they have anything. If the same card was offered for a HP workstation that now has official HP W10 drivers I'd go get that before I went to LSI. And yes you might succeed with use of older drivers if you cannot find newer ones. An example: I needed to use an old HP Oxford chipset based PCI card (not PCIe) for serial/parallel cable access under W10, and the HP drivers for W7Pro64 worked fine. That was a surprise.....