Do not put the pump IN the supply line...put it on the return line, otherwise, all of the water must pass through the pump, and that may not work well. I'd probably replace the (generally cheap plastic) drain valve on the bottom of the tank with a nice ball valve, and plumb in a T fitting to return the water to the tank through that. The ball valve will give you a straight shot you can eventually try to crack up any mineral deposits that accumulate in the bottom of the tank, and, being a full-port ball valve, will provide better flow should you decide you want to flush it on occasion. I'd also consider adding another return line or two, so all of the fixtures would get hot. You can control the pump with a timer, a demand switch, an occupancy switch, or just let it run.