Best Of

The separate water heater/boiler idea is very good. When one goes, the other isn't affected.

Be sure to have the water heater sized to meet your needs, as well as any boiler replacement requirements. This is very important. Be sure to do this. Sizing your replacement boiler is the most important thing when getting started.

From your pictures, it looks like one boiler is much older than the other. If you're able to, replace the older one first, keep the setup you have, and add a crossover as @EBEBRATT-Ed mentions above to allow for both boilers to be used if ever needed.

A combi boiler doubles as a tankless water heater. That configuration is becoming more popular. But you can also use a storage tank with a regular boiler. The tank is connected to the boiler kinda as if it was a separate zone. It's really pretty simple. If you're already going to have any zoning, the tank would be an incremental addition.

if the walls are just air then my favorite retro insulation is blown cellulose fiber. With minimal disruption to the interior or exterior (your choice) you can get good R value, good air intrusion reduction while still keeping it “breathable”

A home as old as yours will most likely have horse hair laid plaster and you do not want to ruin it by breaking it. I agree with ethicalpaul as blown in cellulose is what you want for insulation. You can purchase 3M window insulating kits for seasonal use. Upgrading single pane windows is best left for the summer months to replace caulking and putty to improve window sealing against drafts and replacing sash weight ropes.

For plaster and lath and old windows, Jamie here is familiar with that.

He may be busy with Christmas now.

I did not see a laundry room in the plan?

Blown in Havelock wool is also a good breathable option that’s nontoxic.

https://havelockwool.com/collections/store/products/blown-in-insulation

I have sent you a PM, @lchamb .