Gas Heat Boiler and Hot Water Heater Selection, Please Help

Please help! try to do gas conversion for a five unit, it has only one big oil boiler now, and the heat and hot water is included in the rent, cost me $2000/mo during the last winter. my plumber suggest to convert it by using Navien Condensing combo gas boiler, one for each unit. does anyone have used this system before? Any recommendation would be much appreciated!