Need help sizing a circulation /hydronic pump. Please help!?

Have 75,000 btu boiler, cast iron, with 1" supply and return. Using with a 79 gallon indirect, hot water heater /storage tank. Co. says flow rate for coil is 5gpm . They do not list pressure loss at coil. I am using 1" heat pex for supply and return to HWH. About 12'. What circulator should I use, please? Such as what size, series, Taco,Grundfos, Wilo ? Thanks!......................