Hi Ben, I have seen attempted installations such as the one you are describing end very badly, its OK to ask for advise here but I would be very careful attempting to do this yourself if you dont have the experience or are appropriately licensed. Your systems sound very common and a basic high temp bronze body circulation pump such as a 10 AMP Grundoss or Wilo should suffice. Pumps are not the ideal solution for wetback circulation when natural thermal movement is not available, the most common way to circulate would be to use a pulse valve. You can view more information on this here >> http://www.plumbers.co.nz/forum/codes-standards-health-and-safety/10/wetback-installation-guidelines-how-to-install-a-wetback/1137/       

I think the first thing you need accept is that any pump you install will most likely have to be replaced between 2-5 years. I have done some of these in the past and the distances you are describing should be no issue at all for a 10 AMP, just make sure you get a bronze body one that can cope with the temps (avoid traps when running pipework). We used a solarstat to control the pump but there are several controllers out there you can use, make sure the sensor pocket is on the return. Yes you will still need an open vent, make sure you place the pump correctly to prevent overflow when the pump fires up, the shorter the circuit the higher the risk. Also just set the pump on the slowest setting usually 1. In regards to your HWC is it an open system or will you be using a heat exchanger? I wouldn't worry too much about corrosion to be honest but if you are concerned you can retrofit the HWC with one from the top by using a T or asking your cylinder supplier to weld in a connection.