The HGR15 sized rails are rated for some pretty heavy loads already.
Spacing the rails and bearing blocks further apart reduces the leverage that the spindle loads have over them when loaded in X and Y. At what spacing it becomes the mounting materials which are deflecting more than the rails becomes the key question there. HiWin do publish deflection loadings for their rail blocks.
Ideally you’d want flat face contact for the spindle holder to the Z carriage, or any tramming adjustment. You can rotate this, ideally with eccentric or set-screws pushing the spindle mount to rotate into X alignment and then lock it in place with the main bolts.
The Y alignment is harder, either you rotate the X rail mountings at the Y plates or have to do things with spacers & shims. Like on the Shapeoko there doesn’t seem to be any obvious bolted joint that allows for Y alignment of the spindle bracket.
You may have trouble using all the Watts of the spindle even in a super-Nomad. Unless there’s a big saving buying it all in one go I might grab a CW3000 or clone first.
I believe Mechatron are decent enough to specify the required flow rate and supply water temp for their spindles.