Won't need real winter gear for CA coastal riding.
I got a second set of fenders, cut off a couple of maybe 10" pieces from the 2nd set and pop rivetted them to the existing fenders, thus making them longer. You obviously would cut off the silly little mud flaps which come on those fenders. To the bottom of those, I added mud flaps made of stair tread material to reach about 2" from the road. The mud flaps flair out to 4" wide at the bottom. I bolted those on to make replacement and adjustment easier. You'll have to fiddle around some to get that stuff just the way you want it. Plastic fenders respond well to careful reshaping with a heat gun.
For clothing, you want a couple pair of tights of differing weights for different temps. Pearl Izumi are good. For your top, thick poly undershirts, one SS, one LS. Craft are very good. Arm warmers of course. A couple LS jerseys of heavier than usual weight. For water, just a wind shell. You really don't want waterproof. You need to let the rain in for cooling. Voler wind shells are just right. Wind vest, too. This gear gives you many useful combinations, from SS undershirt, SS jersey, arm warmers and wind vest, to LS undershirt, LS jersey and wind shell.
For the feet, regular cycling shoes with wool socks are fine, with neoprene booties over. To keep the water out of your shoes, you buy dry suit leg seals, online or from a dive shop, and put them on your naked ankles, bell down over the tops of your booties, tights zipped down over the tops of the leg seals. Dry feet!
Gloves are the hardest thing. For cold rain, the best thing I've found are Giro 100 Plus gloves, though many folks like leather over thick wool gloves. There are many decent gloves for warmer rain. Craft makes the ones I've liked the best. I have 3 pair of their gloves of increasing thickness. One of the most important things with gloves is being able to get them on and off when they are soaking wet. They will be soaking wet. "Waterproof" is useless.
I've been riding in the PNW winters for 25 years and the above works best. I've ridden 75 miles in a steady 36° downpour with the above gear, except that I had winter MTB boots instead of booties, but that shouldn't happen that far south.
A heavy LS jersey is great if you can find one. Avoid anything "softshell." They look nice on the rack, but are useless for real riding as they absorb water. Get Hiviz or at least yellow stuff for your top.
Run serious lights in daytime, 300 lumen blinky in back, 200 lumen blinky in front.
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