what mortar over radiant heat?

Don't do the thinset in one step unless you're really confident of your abilities. The mesh makes it more problematic. Your two step plan sounds fine.

Don't use the ready mix. It's not that much work to mix the powder, and it's a lot cheaper.

Definitely use modified thinset (Home Depot's is fine), and use latex modified grout as well.

In lighter traffic areas, and with suitably large tiles (minimum 6") I like Wedi Board. It's a Dow Corning foam with a modified acrylic cement coating over fiberglass mesh on both sides. It's really easy to use and install, adds insulation under the heating mat so you get more heat coming up rather than heating the slab/floor, and some electric mat manufacturers specify its use.

Thanks, that's great. If I've already put down 1/2" durarock, is it worth pulling it up for the Wedi Board?

I'm not sure on the area we're talking about, but I'd tend to doubt it. The Wedi Board is about twice the price of Durock, and what with labor and all, the payback time would probably be pretty long.

Well, it's all kitchen, and the labor is all mine. It's also over an insulated wood plank over a basement. And it's all high traffic. I'm told it's a waste to put down radiant flooring in low traffic areas. I'll keep Wedi Board in the back of my mind for when i do the bathrooms. Thanks again

I'm not sure what the radiant heating has to do with the amount of traffic. A master bathroom is pretty low traffic and a great place for radiant.

The Wedi Board is a joy to use on second floor bathrooms. You can walk up the stairs carrying four sheets easily. Try that with cement board.