Pick out your trim first. Different brands have different valves that aren't compatible (usually) between the brands. The specs on the trim will tell you your valve options. People on the forums recommend valves with stops - these allow you to turn off the water supply at the valve rather than having to turn off water to the whole house. I'm not sure if you can purchase each trim element separately, but my thought is that you probably can without too much difficulty in compatibility issues, even between different brands. Threads on plumbing are standardized, so buying a separate shower head and arm shouldn't present a problem. The valve trim will still need to be compatible with the valve, though, even if it doesn't match the shower head and arm.

Discontinued items are alright as long as you're positive it's what you want; you won't be able to return it as easily if you change your mind. You might also have a more difficult time finding replacement parts if you ever need them, though usually the innards are the same between different faucets within a brand.

Thanks, as usual there are way more things to know about this than I realized, so I'm a bit overwhelmed!

If you want durability, quality in valve construction, you;re not concerned about the "latest thing" style-wise, and you want a great price point...AND you're looking for pressure-balanced...

Look at the Symmons Temptrol line.

The valve bodies have been around for I can't even tell you how long. The valves are well-designed, spare parts if needed will always be around, and over the years the few times I've needed parts, usually for worn o-rings, Symmons has sent me replacement parts for free.

They are sold at the box stores (some are at least), and they are packages valve body and trim kit together, plus some include the shower head and tub spigot. Depends on what setup you need; shower, or shower plus tub.

In general, the entire package can be had for $80-$125 give or take.

For economy and performance, I recommend them without hesitation. If you want the latest-greatest style, look elsewhere.

A lot of shoppers will install this set but replace the shower head with another. Shower head threading is pretty much universal, so mix and match is simple.

Thank you, mongoct, that's very helpful. I made another post above about using a Danze set-up I got for free, vs buying a low-end Grohe, which I was under the impression is very reliable.

I can get a recently discontinued Grohe trim kit and a Grohsafe valve for the same price as you said the Symmons would be. Is the Symmons better, less good, the same? Thanks again, I truly appreciate it!