Looking at a gas hot water heater to replace a Marathon electric that is going to go to a rental house of mine with a lower element in its heater that has failed and is so carroded I can't get it out without breaking the tank.

Can a gas heater go in a closet that held the electric? Are powervents more efficient? Is there any point in going to a tankless? Are Vertex heaters more $$ than 2 60 year olds would ever save in efficiency?