This is what I would look for in a heater shatterproof, Shuts off automatically and Fully submersible.
Visi-Therm Stealth Heater. Sleek, inconspicuous aquarium heater design incorporates accurate, "shatterproof" performance. Reinforced plastic casing protects the components against thermal and mechanical shock to eliminate shattering. "Shuts off automatically" when out of water; restarts automatically when in water. "Fully submersible". Temperature select dial and aluminum cylinder distribute core heat evenly and efficiently with ±1.0°F accuracy. Easy to set and install. UL-Listed.
This one looks good. I would stay away from ones that have a sperate thermostat as I overheated a tank when the thermostat was moved during cleaning.
As for wattage you may be better off with two smaller heaters over one in case one should fail. The more wattage means the heater will run less using more electricity. I ran my FW 180 on one 200 watt for years.