Here is a chart with the neu kv info.
I personally would get a 1515 and aim for about 35-40k max motor rpm, but there are much more knowledgeable people than me on this board. 35-40k rpm divided by 18.5v gives you about 2100kv. So I would probably do a 1y, unless you planning on upping the voltage someday, then maybe a 2.5d. The 2.5d should run pretty well on 6s, while IMO the 1y on 6s is pushing it. But the MMM system is supposedly going to be a 1y with a 6s max, and castle claims theyve run it very successfully in testing.

Apparently I type slow, 2 people posted since i started this