Separate heat and hot water systems for two apartments

Hi, I have a house in Gowanus & would like to get mechanicals out of the cellar. This is a 2 family house and I am considering having separate heat and hot water for each apartment, with mechanicals residing in each apartment. Building currently has gas boiler & water heater in cellar. I am familiar with the standard gas fired steam boiler & gas water heater w/large tank, but know nothing about other options. As far as changing to separate systems (I have checked old postings in the forum) I am super confused about how separate systems work & options. We are considering a gut reno, but would like to avoid if possible. What is the least expensive system (in installation & material costs) that will give us separate mechanicals? What is the most cost efficient & low maintenance system? Thanks!