First on the spindle, I like the idea of a spindle instead of a router, especially since learning that a fair amount of people have issues with the collets on the trim routers. Any concerns on the limited RPMs on the one you linked to? I thought you’d want to be able to take it up to 20k RPM for some bits.
The 12k RPM is low and where you need the higher RPM’s is for smaller diameter bits like 1/8" to 1/16", but 8k RPM is not going to make a substantial enough difference to outweigh the benefits of having a quiet, rigidly mounted, concentric spindle in the same pricepoint; really for using small diameter endmills/engraving in most materials you want to be moving 60K RPM, the problem is the very high RPM spindles will have no torque at the RPM’s required for 3/16" to 1/4" end mills.
Staying within a couple hundred dollars, I have not seen the perfect option for smaller CNC’s outside of building your own spindle. Personally I would like the same formfactor as the spindles I linked with a good torque curve between 15k and 30k RPM; that would be a great general purpose spindle; in the meantime I have not been able to beat the price for performance of the 12K spindles and I have milled wood, carbon fiber, and aluminum at 12k RPM with good finishes.
I hadn’t even looked into linear guides! Are they ok to be turned on their side? Do you recommend these for the Z axis as well? Nearly every Z axis I’ve seen has been a linear rail like this2.
Yes, you can put them in a flat side by side configure or have them facing away from each other. Each axis should have 4 carriages and 2 rails. You can go look at the Multicam as an example. Most of the modern professional machines being built today use linear guides on all linear axis, that is because linear guides are better in every way (except cost) from performance to ease of implementation. The Multicam in woodshop and HAAS in machine shop are all linear guides.