Maintenance prior to dropping everything back in the car

Being gold colored freezeplugs, they are probably brass which means they were replaced 4-5 years back as you say. I'd leave them in. Any crap will only get into the cooling jacket so it is not a disaster, but I would personally bet on them being OK. 30-40 year old core plugs is the real problem.

Your clutch disc looks to have some life left in it for general street use. There are a few slightly 'burned' spots on both the flywheel and pressure plate, so it is not perfect but OK for general street use. I've put similar parts back in for a race or 2. As a minimum cleanup, I'd use some Scotchbrite to take some of the shiny surface off (do the whole surface on PP and flywheel, not the clutch disc) and then clean everything thoroughly with brake cleaner; don't spare it. It'll work as is if you wanted to wait for the time when due for another clutch & PP.

I would also not hesitate to have the flywheel resurfaced; it shows some definite wear. (Actually, I would just put in new clutch and PP, and sell this clutch and PP just to get things fresh but that is just me.)