You said preheat tank, so Im assuming your using it for domestic hot water. Wraping the 3/4 inch copper pipe around the flue, will work, but Im afried it might work too well, and get way too hot.
You of course would need a pressure releif valve on the tank to blow off the steam that you may make. As far as the number of turns around the flue, if used for domestic hot water should be few. I guess one would have to know the average temp of the flue where the copper would be wraped. you will not have anyway to control the temp of the water if you need the wood stove to run 24/7
This type of application would be better to use for heating with the use of heat exchangers, or wall mounted fin tube, with expancion tank and again pressure releif.
I would not use anything but copper or gal. pipe all the way to the main hot water tank, than you could use the pvc pipe through out your home. As far a sodering a joint, 50/50 soder will not melt as long as there is water in the pipe. Or you could use silver soder, or 95/5.
when you are trying to thermosyphoning any thing, make sure your return always has a downward run, and the supply has a continue uphill run. no loops.
