First, please read this: http://www.nraila.org/statelawpdfs/CASL.pdf
This link tells you what you may and may not own, whether or not you must register your firearm, etc.
“ASSAULT WEAPONS”, .50 CALIBER BMG RIFLES AND MAGAZINES
It is unlawful to offer for sale, give or lend any “assault weapon” or .50 caliber BMG rifle. It is unlawful to possess an “assault weapon” or a .50 caliber BMG rifle unless it is properly registered with the state. The registration period for “assault weapons” and .50 caliber BMG rifles has ended. [Does this mean it is impossible to legally acquire an AR-15 in California? -bkt]
If an individual has an unregistered “assault weapon” or .50 caliber BMG rifle after the registration period has ended, they must relinquish the firearm to law enforcement. Anyone with a registered “assault weapon” or .50 caliber BMG rifle wishing to legally dispose of the firearm: can sell it to a dealer with an “assault weapons” permit (for such firearms) and .50 caliber BMG rifle permit (for such rifles); or with both type of permits; relinquish it to local law enforcement (after making an appointment); remove the “assault weapon” or .50 BMG caliber rifle from the state or destroy it. Any person inheriting a registered “assault weapon” or .50 caliber BMG rifle has 90 days to render the weapon permanently inoperable, sell to an approved firearms dealer, obtain a permit from the Department of Justice to possess firearms, or move the gun out of state. Pawning “assault weapons” and .50 caliber BMG rifles is not permitted.
California law defines “assault weapons” in three ways:
• firearms listed on the original Roberti Roos assault weapons law.
• any firearm that is in the AK and AR-15 - type series.
• by the specific generic characteristics.
A further explanation of firearms considered “assault weapons” can be found on the Department of Justice website.
A person may lend a registered “assault weapon” or a .50 caliber BMG caliber rifle to another person who is 18 years of age or over if the person to whom the “assault weapon” is lent is not prohibited from possessing a firearm and remains in the presence of the registered possessor, and the “assault weapon” or .50 caliber BMG rifle is property is in immediate danger and the weapon must be carried for “preservation.”
• a person “engaged in the act of making or attempting lawful arrest.”
• a person carrying a firearm while at home or at his place of business, including temporary residences and campsites.
Assuming I'm reading this wrong and it is legal to acquire or build your own AR-15 in CA, no, you're not getting ahead of yourself. What do you want to do with it? Do you want a match gun? Varmint gun? Carbine? Once you answer that, you can buy what you need. Complete (or mostly complete) uppers can be had starting around $270. This may cost significantly more depending on what sort of rifle you want. A stripped lower will run about $150, and a lower parts kit can be anywhere from $60 and up. And you'll need a stock ($50 and up).
Call a gun range or two in San Diego and ask them what the laws are. If the local law enforcement are anything like what we have here in New York, they won't know the laws well and may give you incorrect information, but folks who operate a business that requires knowing the laws will be able to help you.