I work at a HVAC company and Pex infloor heat is the best there is, unfortunatily you want it under the cement floor. We hardly ever put any in the floor joists. Heat rises and putting it in the floor joists would be a waste of money (won't heat lower level) I'm assuming you have a main floor and a below grade basement? I'm not understanding the no ductwork? If you put infloor heat in you still need ductwork for the heating and cooling. If you could of put the pex in your lower level cement you still need the main floor heated. Floor heat in your cement does not heat the upstairs level, it will bring up some heat due to heat rise but not to heat the main level. Depending on the size of house you will need either a electric or gas boiler to run the floor heat. A duel element water heater will not do it. This is an awsome but expensive system to put in but as I said before it's a comfort kind of heat not the main sourse for the house. YOU STILL NEED HEATING AND COOLING. Good luck!
If your house is a slab on grade house then disreguard most of what I said but you will still need a heat sourse for the infloor and an airhandler with backup heat in it, an A/C and DUCTWORK.