Not at all! You see, if you keep the pressure within the system below atmospheric pressure, you could run the system at any temperature at all. For example, if you want to make energy from an ambient temperature heat source, you just keep system pressure around 10mpa. Then the water boils at room temperature, pressure rises, and you can do work. (Of course as always you need a cold sink to re-condense it.) Definitely! You can use any working fluid. Positive displacement engines will always be more efficient than devices that rely on areodynamic forces (turbines.) That's why cars have Otto engines and not turbines.