Dremel spare part as spindle
I have found that Dremel spare parts are available at reasonable prices.
They have a portable motor tool that has a 10.4V motor with a extra spindle bearing and the collet holder.
You can have a look at some of the details on the following Facebook post.
With the DIY nature of the HydraFabber I think having the largest spacing between the kinematic mount contact points would be advantageous.
The rigidity of a twin smooth shaft 3D printer will be limitd if you expect to have any significant tool tip cutting loads on it.
The torsional stiffness of the two parallel bars is going to be low and the closer the two bars are the worse it will get. The further from the bars the tool tip is the worse it will be.