Need to replace corroded supply to radiator

Here's what I got: a leak in the radiator line for a second floor hydronic rad in our 130-year-old home. Both of the supply and the return pipes look extremely corroded so I plan to replace both of them in one go. They are only corroded in the region immediately below the radiator. There is a 90° turn to the vertical pipe and the vertical pipe looks fine.

I spent the entire weekend reading and researching and got a quick education mostly from this website. It's an incredible resource!

The radiators in my home are column style Roccoco radiators from American Radiator Company. The radiator in question has an EDR of about 60 and it's piped with 1 1/4" black iron pipe. The system appears to me (after my research) to be an old gravity system that was retrofitted with a B&G pump mounted on the return line. I have been oiling the pump every year but that's about all the maintenance I have been doing. The pump connections have surface corrosion. I also have an old expansion (correction: compression) tank that I'm wondering could be water logged? We've only lived in the house for 6 years but I honestly doubt the previous owners put much effort into maintain the heat system. The pressure reduction valve and the backflow prevention valve both look extremely old and have external signs of corrosion. I've got a lot of green corrosion on the surface of fittings on the return lines too. The boiler looks extremely old, it's a Burnham boiler (210,000 BTU, 80% efficient on label).

After all the reading I've done I have come away with a conclusion I have a whole lot of work to do on this system to get it working better and for it to be a long-term heating solution for my home. I'm also convinced my boiler is extremely oversized. I did a calculation based on the method from Green building advisor using previous gas bills and came up with something like 70,000 to 80,000 BTU heating load. Additionally the total EDR for all my hydronic radiators is something like 366 and I have been able to run my heat very comfortably with the boiler set only to 150° all winter, so that would imply a heating load of only about only 40,000 BTU or maybe 55,000 BTU if unraised the temp to 170 degrees.

However the immediate problem I need to solve is the leaking pipes. Right now my plan is to get the necessary tools to remove the corroded supply and return pipes and repipe it all with PEX C or PEX AL PEX. I want to use PEX because these corroded pipes run through the wood floor joists under the second floor, and it would be a huge undertaking to remove the flooring from above so that I can lift the pipe out of the notches in the wood joists. Instead of doing that I have already removed a big chunk of the ceiling underneath to access the pipes because the water ruined the ceiling.

In order to stop the leaking, I turned off the supply gate valve and also the gate valve that is just passed the backflow prevention valve and pressure reduction valve. This stopped the dripping from the corroded pipe. I also turned off the gas to the boiler.

I'm pretty convinced that I can do the plumbing part of the job no problem. But the HVAC part scares me a little bit.

I have a radiator key and went ahead and opened all the bleed valves on the second floor radiators. They all hissed for a while while they sucked in air. And then they stopped.

My plan now is to go into the basement and open up the drain valve connected to the boiler. But I'm not sure if this is a mistake to do this? Because maybe it will drain the expansion tank too and maybe I don't want to do that?

Anyway If it is okay to do that, I'm thinking that I will drain enough water out of the system to get the water out of the corroded horizontal pipe in the floor joists on the second floor, and I will cut it out on one end and remove the cut piece and then replace it with PEX. Then I will get it all squared away and then refill the system.

A major question about this how to determine what diameter of PEX to use. I understand that modern pump systems only need far far smaller pipe diameters than old gravity systems, but if I replace the old gravity piping on one radiator with more modern narrow pipe, I might restrict flow to that radiator. But then again that particular room has always been sweltering in the winter! So maybe reducing the pipe is a good thing? But if so, should I go with a 1-in PEX pipe then? Or should I go with a more reasonably sized 1/2-in?

I'm also thinking about whether I should run a cleaner through the system and then add inhibitor to the system? But I'm not sure if that's the right thing to do. I know the adding fresh water to the system is bad for it. I'm also really worried about the expansion tank and how to handle that. It's a very old tank and after doing all the reading I've done, it seems like I should probably replace it with a newer bladder style tank? One problem with my existing tank is that I cannot find anywhere that there is a Schrader valve. It might be hidden up in the floor joists where I cannot access it.

Anyway, I understand there's a lot here! Sorry if it's too much information.. But I figure more information is better than less. I even have a diagram of all of my loops that I could post but it's probably overkill. It's basically just an old standard gravity system, I think, that was retro fitted with a pump and an expansion tank.