You're thinking of replacing the whole thing, including valves? That's pretty major. You'll have to select a unit that matches spacing you have or it will impact the finished wall. You'll also need access from the back (a large hole in the wall). I'll bet you can get the existing valves repaired and fit a new head and you'll get pressure back.

You don't need to use a Delta shower head, any brand will fit. Replacing the cartridge on the valve might fix the problem, though, so you might not even need to replace the shower head. You can have one on hand, and then return it if not needed. Where is it leaking? It's also possible that you might only need to replace the washer in the shower head.

Drips from the shower head all night. The cartridge is a component within the valve? My BF would be happy replacing the whole thing but if it makes the job bigger he'd likely fold on that. I know the brand, can I guess which cartridge I'd need? How do plumbers do this? They carry likely spares with them?

Cant see your photos. A plumber would start with shutting off water supply A #1.

Most times a progressing drip leak is a valve stem seat and or orings.

Some older 2 handle NON pressure balanced fixtures you can remove repair the stem , newer stuff has a myriad of different cartridges.

Careful w DIY on this. if your BF is not in the know id hire a plumber.

This is a thermostatic valve. I assumed Delta since that's what you mentioned. It probably needs a new cartridge, and the plumber should replace the seats and springs at the same time. Some plumbers carry a variety with them, some will have to hit the plumbing supply store or well-stocked hardware store if there is one in your town (not Ace Hardware), after seeing what you have. If you have the receipt from when it was installed, you can tell the plumber ahead of time what you have.