Head height is much more of a consideration when it is an open ended situation - i.e., where you pump the water out, but it isn't in a continuous loop returning to the inlet. With a recirc system, the water 'falling' back down significantly aids the water being pushed (or pulled) so the head is more just the restrictions from the fittings rather than height changes. The only issue might be getting the line full in the first place (much more of an issue with heating systems and their much lower water pressure and fully closed operation than a potable system), but not normally an issue where you have the street water pressure that is more than sufficient to fill a typical residential system. IOW, a MUCH smaller pump would probably work just fine. And, code requires anything on the potable water side to be non-reactive. Things that qualify are bronze, brass (yours is probably bronze), and stainless steel). Having too large of a pump can also literally create pinhole leaks in the piping as well.