Buy big stuff now.
When I had the 25 as a frag tank (I run my frag tanks like a personw ould run a real system) I bought things that would work for a 70. Now I am running that 70 and am buying things that will run a 120+. It is maybe costing me more in the long run but it gives me the luxary of learning a great deal about all types of equipment and what works best for me along the way. RO/DI, water chnages, halides (on the 70), T-5's (had them on the them on the 25), sumps, skimmers, fuges.
My nano cube is still very difficult to maintain in terms of water quality when compared to the 70 gallon frag tank but all the time spent will help me have a great display tank one day.
I made a deal with myself when I started this hobby
If I cannot keep the nano with three corals and one fish alive, then I wont accept the responsibility of a larger tank with more life on my hands.
Think of it like this. If you have one dog and you cant get its pen clean and keep feeding it and taking it outside to take a crap. Then would you get 12 dogs? Not without inovation and time spent upgrading with a doggie door and food systems and other things. Even with all those items 12 dogs takes more time than 1.