Replacing 1-pipe steam with hot water

In one word. No.

Don't do it. If -- and it's a big if -- you could reuse the old radiators, and if -- an even bigger if -- you could reuse some of the feed piping, all the radiators would have to be converted to hot water (old vent removed, new air bleeds installed, return opening plug taken out, probably new valves). Then new piping -- it's unlikely that you could use all the feed piping, and you'd have to run new return piping anyway. Then of course a new boiler and pumps and controls. If you went for new radiators... they aren't cheap.

And for why? You might make a slight gain on efficiency, but I doubt very much that it would be anything near to recovering the investment. You won't make any gain on comfort; if you don't need to change the temperature on a room by room basis daily you can adjust the room to room balance with the vents. If you do, you can get thermostatically controlled vents.

Moving a few radiators is no big deal -- but if your contractor is recommending changing from steam to hot water, he doesn't know what he's doing with either one. Find someone who knows steam to make the change.

So again, bottom line -- don't do it. Waste of big bucks for no gain.

Agree with @Jamie Hall.Also, no reason to use heat exchanger for basement unless you have water loop piping that's above the boiler water line. No brainer.

If you do not have a low water cut off to protect the boiler you need one!!!!! The low water cut off should be wired as the first control to enable it to shut down the boiler in the event of a water loss to prevent an explosion from the shock of cold water reaching the boilers steam chest.

You can hang steam radiators on the wall if you want heat the space in the basement and I am sure you can buy radiator covers and paint them a nice color.

I personally think you should keep the steam and i do use heat exchanger's for the water loops. I feel it is a cleaner, more professional job than what we around here call a 'dirty loop'. Its kind of a Cadillac or Chevy thing.

As long as the LWCO is operating properly you should have no worries. You should flush your LWCO at least once a week to be sure that there us nothing building up in the float chamber. If you are using only a probe-type electronic LWCO then make sure to test the unit monthly by allowing the water to drop below the probe to make sure the burner shuts off.

You can get a hot water coil for the EG/PEG boiler if the left end section has a proper opening. Use that as your heat exchanger if you want to add that layer of safety. You can add a limit control to stop the burner at 180°F for that zone only. This way if only the basement zone calls for heat you won't be making steam