it is not totally clear what will be mandatory part of the Upgrade to Elite, so what I would be concerned about, is if you buy the Z-20 Z assembly now, that it may also be a part of the Upgrade package.
Regarding the spindle itself, since it is not part of what Onefinity offers, you could buy it and get familiar with what is necessary to wire it up (VFD, spindle cable, etc.) so you are ready to use it earlier. Unfortunately the 80 mm spindle mount for standard series Z-16 Z assembly will not be re-usable on the Z-20 Z assembly.
There’s a different set of cables for the Elite so you could save money and only buy the Elite setup from PwnCNC which you’ll have to wait to use. I see they also try to coordinate with Onefinity on the Elite shipment time, maybe they’ll do the same with the upgrade kits.