Greetings all - I'm a homeowner that's been lurking here for a few all seem to be very knowledgeable and helpful.

After a bad experience with a HVAC contractor in my prior home, I decided to get more "involved" in the selection of a new system in my new home. I have a few questions and would appreciate any advice. Nothing technical, just conceptual, as we will be using a contractor for the installation.

Current Data:
1700 sq ft ranch in central NJ, built 1958
Full finished basement
Half attic
A/C - window units
Heating - Hydronic system with 3 yr old Burnham boiler (nat gas) with old copper piping and old fin-type radiators.
Summer temp - 90's
Winter temp - 20's

My wife and I want to replace the window A/C units with central, and figured it might be a good idea to evaluate the heating system since the pipes are very old. As a matter of fact, there's evidence that one pipe burst and was replaced prior to us moving in last year. Replacing the pipe is not an option due to the high cost. These pipes are behind soffits and are not easily accessible.

Normal ducts will not be possible, so I've settled on a HV system. I'll use Spacepak as an example here since I've read every piece of documentation published on the web.
Spacepak makes a hydronic coil add-on that bolts to the air handler.

Question #1 Does anyone with HV/Spacepak system experience know if this add-on coil can handle heating an entire home efficiently, or is it designed to be an emergency backup for heat pump systems?

Question #2 Given the increased efficiency with modern heat pumps, would a Spacepak/heat pump with hydronic emergency heat be a viable alternative?

Any comments/advice is appreciated.