[Jumbosock]

You don't need the 18 studs. Maybe 2 or 3 in case u pull one out and have issues with it. Even if u pull one out (they can stick to the nut) you can reuse them but it's nice to have spares. The 18 nuts are supposed to be one time use so it's a good idea to get new ones. Don't throw all the old nuts away. You might need one in the future.

The header to exhaust nuts and bolts can be reused (so no need for new ones) but I did have two of mine rusted so I did buy new ones. I actually don't like the OEM ones because they use funky star fittings.

The 8 reinforcement plate bolts are also supposed to be a one time use item and so you are correct in getting 8 new ones but I would just reuse my old ones. Supposedly they stretch a tad for proper torque fitting but I think that's a bit excessive. If u don't mind the price. Get them. Again, don't throw away some of the old ones in case u need one later in life.

The air valve control gasket is a little thing. I would reuse mine but if u wanna be safe get a new one.

Everything else on your list I would say is a must to get new.

If it's been a long time since you've gotten post cat O2 sensors, this is a good time to replace them. They go out so easily it would be a good idea to replace them now.

Also when the header is out is a perfect time to replace the ring seal on your constant pressure valve. It's a 5 dollar item and way easier to replace without the headers in the way. Plus they tend to go bad. Don't get a new CPV. Just replace the seal.