A tankless water heater will be mediocre at supplying both domestic hot water and hydronic heat.
- Quietside makes one that is decent.
- But save your money and get the Triangle Tube Prestige Excellence (a boiler that heats water) and you will be much happier.
Yes,copper is the best, but that cheesy plastic stuff works well, too.
- Caleffi has a website that is chock-full of info about this and anything else you could imagine hydronically. I also recommend their stuff over any others.
Runtaland many others make flat-panel radiators that outperform any other hydronic heat delivery system, and they are beautiful.
For more information:
Read Florian Speier's Q&A "Can I configure an electric tankless water heater to run a baseboard hot-water heating system?"