I wasn't sure from your post if you are in a remodel mode or just replacing what is there. If you aren't ripping tile out to replace the valve, I think you just want to change your fixed showerhead to a handheld? If you are retrofitting, and not wanting to spend much money, I did this in the kids' bathroom. I got an all-in-one from Danze (okay, not highly rated here, but it works well so far, was easy to install, and is pretty handy). The only additional piece you would need is replacement trim for your volume control (the water control thing). What I am learning is that not all trim fits all valves -- in fact, they tell me you have to stick with the manufacturer of the valve. So check that and maybe look at what that manufacturer has by way of handheld shower fixtures. I'm just learning about this stuff myself (and will definitely get the thermostatic valve that astridh recommends for our master bath remodel). So, if I'm wrong, someone else can correct me. Here is a link that might be useful: Danze handheld shower

When I wrote "For the valve; valve body and compatible valve trim." I have "valve" followed by a semicolon, I meant for what followed the semicolon to be a list of "valve parts". So for a complete valve assembly, you want the rough brass valve body itself, which gets buried in the wall, and then the pretty chrome valve trim that you see and touch inside the shower to cover the valve body. In your photo you show the brass valve. You're right, that is the "valve body". You then want the pretty chrome (or other finish) trim to cover that valve body. With the valve body and the valve trim, you'll have a complete valve assembly. As you wrote, a lot of websites will have you pick out the valve trim that you like then refer you to the valve body that goes wit that trim kit. Sorry for the confusion!

This what I like to do in all my bathrooms. This one is a remodel in my previous house of a bath shared by guests and kids. It isn't the budget option, but provides a lot of flexibility and function. Thermostatic valve with volume control and a diverter. This way you can adjust the temperature and the volume that comes out of the showerheads. A small kid might not want as much water pressure as an adult. I like to have a good wall mounted showerhead and a handheld showerhead on an adjustable rod, that also functions as a grab bar (Jaclo). Having the adjustable bar allows younger kids to have a lower shower and is great for cleaning the shower, washing a dog or filling a bucket. Handhelds water pressure are never as good as that from a good wall mounted showerhead (Speakman Icon), which is why we install both.

We used the Kohler HydroRail system, which has both a fixed shower head and a handheld on a pole that attach to a single standard height outlet, and that's worked really well for us. Coupled it with the Grohe thermostatic valve and so far so good with that after a year--I love the separate controls! The HydroRail has a diverter to the handheld built in, so we just have two knobs: temperature and volume. We picked it specifically because you can adjust the angle of both the handheld and the main shower head. (They make four different versions depending on what kind of main shower head you want--ours is just a standard one.) We use this in a shower with internal dimensions of 32x42, also in CA so everything is 1.75 GPM. We did mix and match Grohe and Kohler in chrome so it worked fine--not sure how their brushed nickel finishes compare. Grohe does make a similar shower system but we didn't like it as much.