If you just have fish right now, you really dont need much constant monitoring. You dont even really need a controller like apex for that, just a controller for your temp and do other tests manually. However getting the apex controller, you will be able to bring that over to your larger tank so I think that's worth it if you want to drop the funds for that now.
But if you are thinking of corals later (and they are easy to start getting into) then lights are going to be your next biggest purchase in terms of $$. I think even for softies it's good to have good lights but lighting is a whole other discussion. Good lights allow you to expand the type of corals you want to keep. You can get by on cheap lights but I'm betting If you stay in the hobby long enough you would wish you would have gotten better lights from the get go!
It doesn't take much to get into corals, you could probably throw some mushrooms or a leather in your fish only tank right now and get the addiction started lol!