I would spend #25 and get Salifert Alk and Calcium test kits, but only if you have corals. If not, then who cares. Nitrate and Phosphate test kits are no big deal for just fish either. Salifert test kits are inexpensive, accurate and a good base to have around.
Do you have a refractometer? If not, get one ASAP.
If you have coral, then focus on salt and change some water for now. If not, then ATO might be a good idea next - you can just a brute trash can for now, or something cheap. If you don't have a way to get RODI, then that might be a good idea before a ATO.
Tunze ATO is $200, so you can dump in water for a while if this is a hard cost to swallow. It is worth it, but you can use that money elsewhere for now. I never minded dumping in a bit of water a day since it kept my eyes on the tank while it was new.
In summary....
Coral: Salifert Alk, Calcium, Nitrate and Phosphate Test Kits... refractometer if you don't have one.
Fish Only: refractometer if you don't have one.
Once that is done, get a way to get RODI and then a ATO. ATI T5 can be next but only for coral since fish do not care. Refugium after that - pods and stuff can go into the tank too. You can skip buying pods and stuff for your refugium and order a 10 or 20 lb live rock bundle from the Gulf and get lots of nice microfauna and bacteria - this is always a good idea, IMO.
For anything: forget about pH. Just not important nearly at all and especially not now. I have not really checked mine in a decade except to really tell people what it is and that it doesn't matter. You can do more to keep your pH in line by airing out your home regularly (the real root problem) than buying stuff to fix it.