Back when had the JBJ Nanocube 28G I bought the Tunze Osmolator. IMO it is the best investment I made for my tank. That thing is so precise and is now running on my current tank. After 2 years I spent £18 replacing the water pump, but thats fine considering the little pump brings RO water into my tank from about 5ft below and then travels another 8ft across to the tank, so its worked quite hard.
I have a GHL Profilux P2 EX PLUS controller on my tank, but I trust my Tunze Osmolator so much that I wouldnt go without it.
I only need to add water to the top off bucket once week. I reply on evaporative cooling, else I reckon I could have gone 2 weeks. Plus my tank is Rimless/open top.
So all in all, after, lights, pumps, skimmer, I would buy an auto top off. Put off buying a few corals and fishes for a month and spend the money on buying an auto top off. It makes enjoying your tank on a daily basis that much better. No need to top off every night. You can go out for a weekend (or a whole week if you use a larger top off container) and dont need to worry about the tanks water dropping.