Are you planning on installing the heat exchangers and pump etc, yourself? My guess is that the tank will be half your cost.

YUP!!!!
Very, very wise move.
As you've probably read, an average family of 4 (who's average!!?) highest electrical use is heating water.

I have one and I wouldn't change it for the world. I don't use much hot water, but it's heated free. Oh - and my back-up in case I have no sun or it's very cold in the winter, my energy efficient, regular water heater kicks in. It's set pretty high, so that's very seldom.

I'm in MD and can't speak to CA vendors, but well worth it!

There are myriad ways to make hot water from the sun's energy. I believe that most solar HW tanks are larger and better insulated than conventional tanks. That would be no real problem. I think that these tanks have limited heating capacity compared to those that are not designed for solar. That would mean that recovery time might be much longer.

This house needs a lot of work and I want to make it livable first. That requires some major plumbing repairs including running new supply lines from the hot water heater to the master bath (fortunately they are only separated by a wall) and while I'm at it, the HW heater appears to be very old. However, to get plumbing to the roof, I need to go through the 2nd bathroom above the garage which while in need of a remodel is not critical so I would like to leave that for a second phase.

The back of the house faces south and has a big, roof surface that would be ideal for solar. Since we are in southern California, it's would be a shame not to take advantage of it.

I'll be hiring contractors to do the work.