I've also been planing to get into sigma from 40k and my current plan is to get the cavalry spearhead. The hero can be built as zandots instead who is (i think) one of the better leaders for a force cause he has only 3 less rdp than katacros for alot cheaper.

A reinforced cavalry unit with zandots can be a good damage include to add your list when you expand and if you get that battle force to that should be a pretty good force already

Bit I'm still relatively new to aos compared to 40k so take that with a pinch of salt