This spindle looks like a steal @ $100. The seller also offers a complete engraving router with this spindle but it lacks
variable speed except for the step pulleys. It needs a 24v power supply 5A minimum.

Question: How difficult would it be to make this variable speed ? Some cheap plug-n-play board available ?
Keep it simple if possible, electricity is black magic to me.

The ebay listing:
ER11 Spindle FOR Sable 2015 SPD ER11 Engraver MILL PCB'S Engraving | eBay