I bought a car without a title when I was in college and I basically threw the car away because I couldn't get a title. I didn't have any idea how to get a new title and the seller was absolutely no help. So I would be disinclined to buy a car without a title. On the other hand, I'll bet there are people much more savvy than I who would be perfectly comfortable doing it.
I would think that getting a duplicate title or even retitling the car would not be that difficult, especially if the car was always a California car. Any paperwork would likely be helpful.
Here's my story. Even though I am the original owner of my car, I had lost the title during the time I took the car off the road and finished building it. All I had was a notice to renew my registration. When I went to title the car (now in a different state) the lady at the DMV said that having that notice was a big help to her (I have no idea why other than it was evidence that at least I was the owner at one time). As I remember, there were other options I could have pursued to get a title if that one had failed and if I had to, I could resort to the way people who build hot rods title their cars (after all, most of it was new: frame, motor, tranny, interior, just old body).
I think a lot of complications like this are driven by the length of time since the registration was renewed, and in my case, the fact that I was now in a different state. Given the car culture in California, I would expect that you would be traveling a pretty well-worn path.