2008 370k P0300-P0304, P1116, P1121

We have 3 Prii - 2008, and 2x 2005. The 2008 is my son's car and has 370k on it (original traction battery!). We are taking care of issues to get it running better and passing inspection. It has the original struts and the front are quite bad.
My main question is about the engine. It is throwing P300, 301, 302, 304 codes, and the engine tends to run rough. Since these are misfire codes, and I know that I've never changed spark plugs (we bought it with 200k), they certainly need replacing. Heck, they may even be the original plugs. I'm guessing that bad plugs could cause the misfire codes?

Also, the P1116 and sometimes P1121 codes- they point to low coolant or the coolant valve. I have topped off the coolant, and the 1116 came back. So, I assume the valve may be ailing.

I think I'll do the plugs, and see if that clears up the misfires, before diving in to the coolant codes. The engine has always used oil since we got it- we have to watch it, but we have been good about it. If the engine is on the way out, I don't want to throw good money/effort after bad.

The engine itself seems a bit noisier than the others- maybe water pump? Maybe the timing belt is about to go? I need to probe with a stethoscope to nail it down and AB with our other Prii.