Hello, welcome to our forums!
Firstly, if you want to take checking salinity to the next level, you could look into refractometers. They will do the same thing as an hydrometer, but they are more accurate.
I would definitely recommend a heater, though, as the tank needs to be kept at about 74-81 degrees F (IMO
). You will also want to prevent the temperature from dramatically fluctuating, so a heater would be handy in that circumstance.
For your size tank I would say that a sump is not a necessity. It does, however, add a few benefits.
1. More water volume. The extra water volume is more forgiving if you make a mistake.
2. Using a sump is a good form of filtration. For example, you can partition your sump so one of the sections is a refugium. In a refugium you can house kinds of macroalgae like chaeto. Macroalgae will soak up some of the nitrates in the aquarium's water.
Also, I just wanted to mention that there is a possible downside of using wet/dry filtration. Over time, I have heard reports of wet/drys becoming "nitrate factories" (the nitrates can build up in the filtration system). However, I have also heard that if you take care to clean the system, you will not come face to face with this issue.
I hope this helps, and if you have any other questions please feel free to ask.