What you have is a boiler that heats water to steam. Many boilers systems only heat water then circulate the hot water. These systems use baseboards or infloor loops. A steam unit uses radiators that are big and bulky. Steam takes up more volume. They are a higher maintenace system but the efficiency of the system is still determined by the boiler. These systems were used sometimes because steam was purchased from the utility and they didn't need a boiler in the home. When some utilities quit selling steam the homeowner intalled their own boiler to prevent a major rework. A steam system didn't need a big circulation pump like a hot water system. Bottom line check the boiler for its condition. If it is in good condition then ask yourself if you can live with those radiators. If you want AC then it will be a separate system.