Most DIY CNC kits utilize off the shelf solutions as spindle motors. These are certainly compromises compared to what you might find on a professional grade CNC, but they generally do a passable job. On the Shapeoko 3, your two supported options are the Makita RT0701C and Dewalt DWP611 compact routers. These are both very similar products on paper, but from the perspective of a CNC user there are some subtle differences between the two. Check the video for the full breakdown.
*One thing I forgot to mention is that the Makita has about 1/2″ less reach than the Dewalt.