Greetings turbopwr68! Glad you stopped by. Like you, I started out with a forced air system in my garage, soon to find out that I had way more heat than I needed in the garage. Next, it was off to find a way to get some of that great heat into the house. So on with the boiler project. You may have seen it, but if not, you can find it here:
http://wasteoilheaterforum.com/index.php?topic=208.0I think your use of the built in domestic hot water heater is a good choice. Like you said, its already there, might as well use it.
Pex? Even a non plumber like me was able to work with this stuff. Bought me a crimper on ebay and went to work. A guy I work with calls PVC tinker toys for big boys. I'd probably say the same for this stuff.
Getting the boiler up off of the floor is a great idea. I have mine on a row of cement blocks. I cant imagine having it right on the floor.
Good luck on your project. Keep us updated. We love pictures! Give us a holler if you have questions.