Well I'm glad you like the self pumping idea. A few more thoughts for what it's worth:
For the impeller bearing I think you'll want a stainless steel bearing or it'll rust up pretty quick if you use water as a heat transfer medium.
The impeller will be trying to attach itself to the magnet rotor unless designed carefully. For instance consider using only small metal bits as the magnetic driver to keep the attractive forces down. I'd be afraid of an imbalance in the attraction trying to drag one arm of the impeller into the casing otherwise.
Actually the impeller could also be a slotted disc of thick aluminum or copper. The same eddy currents that make heat would drag it around in a circle and there would be zero static force trying to attract it to the rotor. The slots would add impetus to the water to make it move. Of course you could add fins or bars to it as well.
Using air as a heat transfer medium will require a large "tank" and large ductwork to carry it around. Water would be really easy in small tubes, etc. Also, a relatively small volume of water can heat a lot of air on the other end.
If you can lay your hands on a stainless steel "ring bearing" you could make a torus (doughnut) shaped tank and use dual rotors.
For the tank construction I'd make a male mold out of oak on a wood lathe, attach a sheet of copper to it and spin-form it on the lathe to stretch it to form around the wood form (like how they make bowls). You could even be fancy and have some shallow grooves shaved into the wood and use these to form stiffening ribs in the copper.
After you have the main tank piece formed you simply make the opeings for the tube nipples and solder on a flat sheet for the other face.