I was looking at that Katsu router too. It’s supplied with a 6mm and 1/4″ collet but 1/8″ collets don’t seem to be that easy to get for it. Cheapest I could find a 1/8″ collet including shipping was nearly the same price as the whole router. A 1/4 to 1/8 collet adapter might do the job though, not sure what effect it’ll have on the runout.
Have you tried it out yet? And did you get a 6mm or 3/8″ collet? Amazon says 6mm and 1/4″ but Ebay says 3/8″ and 1/4″.