I just retrofit a redytemp.com unit, really easy and very fast install (about 5-minutes!). Grunfos makes one that has also been recommended. Taco (probably the guts of most of them anyway) now makes one called D'Mand system. If it were me, I'd run either copper or pex as the return line, not pvc. I'm not a pro. I'd definately insulate all pipes, both hot and cold (the cold to prevent condensation).