Steam heat questions

Typically, steam heat systems work with a single pipe to each radiator. Steam travels through the pipe up toward the radiator, and condensate drips down along the side of the pipe. Check your radiators; I believe there do exist two-pipe steam systems, but they are rare. With hot water heat, you usually have two pipes at each radiator: one to supply water and one to carry it away. Some steam radiators can be converted to hot water, though.

Hot water heating is a perfectly feasible option in places where you do not also want central air conditioning. Where I'm from (The Netherlands) it's the standard for, I would estimate, over 90% of heating. You need a heater/pump unit that may or may not be the same device that provides residental hot water, and you want an expansion tank somewhere in the system to stabilize the pressure. Other than that, it's just a matter of running plumbing to the radiators. I would advise against trying to save plumbing by hooking up radiators in series; it's useful to be able to turn them on and off individually.