Hey Tom,
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.
Be aware that the drag chain that is offered with the Elite Series is not wide enough to hold everything if you want upgrade to a water-cooled spindle with its spindle cable, water hoses, possibly retrofitted limit switches etc. (See drag chain width calculation)