Best Of

Please keep your old wood windows. The old-growth wood in them will last a hundred years longer (or more). Repairs to pieces on the windows can be made yourself or by a knowledgeable carpenter.

Re: wood window repairs and improvements, I'd recommend "The Window Sash Bible" by Steve Jordan - great book on renovating wood windows. I would get a set of professional scrapers and infrared lead paint removal tools - available from https://www.solventfreepaint.com/. . It is relatively easy and low cost to effectively air seal wooden windows. I removed sash weights from the upper windows (most didn't operate anyways) added 2-1/2" of foam insulation in that part of the pocket and retained lower sash operation. PM me if you would like more details on what I did.

Single-pane windows can have wooden storm windows added. This will double the R-value of the window and increase comfort. Adding indoor shades of some kind will add another R-1 (or more) to the window. Given that a really good new window is probably R-4 or R-5, this gets you most of the way there and costs a lot less than new windows (which will likely look cheap and need replacement in 15 years or so).

I kept most of the wooden windows in my house (except when renovations required new windows). If I replaced all of the large wood windows on the first floor I would have spent another $30k (or more) on good quality Marvin replacements. That was money better spent elsewhere.

Cast iron would be better, last longer than steel.
Energy Kinetics would be the most efficient. All their parts are easily accessible. Depending on model, there are easy work arounds to keep you up and running while you get the parts. Basically, depending on model, it's either the Energy Manager, which can be put into service mode and still work properly, or the aquastat, which can use an off the shelf Hydrostat with a slight wiring change.

Most important is a properly sized, properly installed, properly commissioned system.

There is nothing wrong with a steel boiler for hot water heat. No sections or rubber gaskets to leak.

Steel was used in the past and lasted a long time. With the way CI boilers are now I have no doubt that a steel boiler would outlast a Burnham