It's been quite awhile since I looked into CA's AR laws, but you might consider two CA-legal alternatives to ARs while you're digging into all of this, to see if they might fit your needs.
One would be a Mini-14 - last I knew, they were pretty much unrestricted in CA, outside of mag bans. The newer models are as accurate as most shooters are capable of, which was a big drawback with earlier ones. They fit a scope easily enough, and the stock design is closer to a hunting rifle's layout.
The other would be the
FightLite SCR, which has some parts compatibility with ARs, and would give you more room for tinkering, accurizing, upgrading, etc, over the Mini-14.
If you really want to build a CA-legal AR, a local gun store might be the best place to start in getting current info, especially if it's one of the more tactical oriented ones.
With AR parts, it's definitely the kind of thing where you get what you pay for, in general. If accuracy, reliability, and durability are big priorities, then don't cut corners on the barrel or the Bolt Carrier Group (BCG). If you can find a barrel that has the gas block already installed, even better - Daniel Defense is one of the best values for all of this, with the barrel/gas block read to go. With BCG, finding a company that properly gauges their parts before assembly is hard, and even top companies occasionally turn out BCGs that don't gauge properly. The only 2 I know of for certain that gauge every single part is Sons of Liberty Gunworks and Modern Armory, but I'd take an LMT or JP Rifles BCG in a heartbeat too.
The biggest reliability reason for all of this mattering, is how the gas system works. ARs have a tiny gas port in the top of the barrel near the muzzle - the gas block goes over that, and transfers gas down a tube into the upper, and into the BCG. Inside that BCG is kind of piston - gas blasts in, the piston chamber expands, and hurls the BCG backward to cycle everything. Any leakage makes that system less efficient, and will give the moving parts less energy to cycle properly. That's why installing the gas block properly, and having a properly spec'd, gauged, and assembled BCG matters so much.
Everything else is secondary to the barrel and BCG, in terms of prioritizing cash, and what you get out of that. Next would be a good open-design trigger, rather than a cassette style one. Geissele's are pretty hard to beat, just decide if you want single-stage or two-stage. This matters for reliability the more extreme the situation gets, especially shooting suppressed, around a lot of sand, etc - the cassette designs have sidewalls that seem to trap things inside the trigger and cause it to malfunction in some situations. With open designs like Geissele and milspec types, the sand and contaminant just falls out.
With lowers, the biggest problem people encounter is things being a bit out of spec with how the holes are drilled. So, try to get something at least mid-range in terms of cost there, and just get a matching upper from the same company at the same time. It will also ensure the aesthetic lines match up. You can usually get them at the same time, often as part of a package deal. A few have full bundles of lower/upper/handguard you can get at the same time. There's value in that, in just making sure things are less likely to go wrong, instead of assembling from different companies.
All the other stuff, like stocks, grip, charging handle, that's more personal choice where economizing matters little.
Good luck with whatever it is you decide to do over there.
