A BCM midlength upper is a fine choice. And doing your own lower is smart. Both can be a little more complicated in your state though.
In CA I understand there is a list of allowed brand names for the lower and/or a list of banned names like Colt, ArmaLite, Bushmaster, etc. And for the upper you may have concerns with the muzzle device but I don't keep up on ban states. I'm sure AgentTikki is leading you to the right people so you end up with a compliant AR. Get it put together in a legal form.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek