How hard is it to add a shut off valve to my water heater?

You can solve this pretty quickly with a few basic parts from a hardware store, and you won't even need to touch a torch or solder anything. Since you alredy have a flexible braided hose with a 3/4 FIP (Female Iron Pipe) connection, the simplest move is to install a **3/4 inch threaded ball valve** directly onto the pipe where that hose currently connects. You'll want to pick up a 3/4 inch FIP x FIP ball valve and a 3/4 inch brass nipple, wich is just a short piece of pipe threaded on both ends. Shut off the main water, unscrew that flex hose, and thread the valve onto the pipe coming out of the wall or ceiling. Then, screw the brass nipple into the other side of the valve, and finally, reattach your flex hose to that nipple. Now you have a manual shut-off that keeps the rest of the house pressurized while the heater stays isolated. Those green, oxidized joints on the copper are the only real risk here. When you go to unscrew the flex hose, you must use **two wrenches**. Use one wrench to hold the copper adapter steady so it doesn't twist or snap, and use the second wrench to actually turn the nut on the hose. If you just pull on it with one wrench, you're likely to put a ton of torque on that old, brittle solder joint and you'll end up with a much bigger leak than the one you started with. Make sure you wrap the male threads with some decent quality **PTFE tape**—the pink stuff is thicker and ussually better for older threads—and maybe a bit of pipe dope if you want to be extra safe. Go clockwise with the tape so it doesn't unspool when you tighten the fittings. Once it's tight, you can keep the valve closed and turn your house water back on immediately. It'll save you a lot of headache until the installer shows up on Monday.

I saw some "full port" valves at the shop and wasn't sure if that's a big deal for a heater line. woud a regular one slow down my hot water pressure too much?

Also, is that green oxidation somthing I shoud scrub off with some sandpaper before I put the new stuff on, or is it better to just leave it alone so I don't thin out the metal? I'm super paranoid about those old joints failing once I start cranking on them with the wrenches.

Go for the full port valve. It costs a tiny bit more but prevents that annoying pressure drop in the shower. A standard port valve has a much smaller opening inside that acts like a restrictor, which you'll definatly notice if you're running multiple taps.

Don't touch that green oxidation with sandpaper. That patina is acutally a protective layer. If you scrub it off, you're just making the pipe walls thinner, which is the last thing you want on old copper. Just use a damp rag to wipe off any loose grit so it doesn't foul the threads. Also, check the rubber gasket inside that flex hose—if it's flattened or cracked, grab a new hose while you're at the store.