74.5� is quite acceptable but don't let it drop much lower. It is better to turn it up 1 or 2 degrees in the winter as the ambient room temperature also effects the water temperature.
Pardon me for being a bit blunt stan, but you are a bit of a scrooge using a 2nd hand heater, a new one wouldn't have cost much more. It is also false economy - did you notice the price of fish nowadays?
An 2nd hand heater should only be used where 2 or 3 heaters are needed in a biggish tank. A heater has a limited lifespan which depends on how it is treated. The airstone should be placed near or under the heater, this will help to evenly distribute the warm water and helps to keep the heater cool thus prolonging it's life.
Welcome back to the land of tropical fish and long may you stay. It is always a good idea to keep a couple of small tanks handy in case you or/and your wife feel the urge to breed a few (fish).