Last question first. IF you have adequate gas line, AND installing a stainless steel vent are doable, then a tankless might work. "ECO" can mean several things depending on the water heater. It stands for "energy cut off" and is activated by excessive hot water, failure of an FVIR system, or some other condition requinng complete shut down. NOT all ECO failures are resettable, but since we do not know which one you have cannot tell if it requires resetting or relacement.