If you keep things simple, it won't be super-expensive.
What exactly is causing you to have second thoughts? You can add things slowly, over time.
The only thing I would do is buy a used AC70, modify it to be a refugium and add it to the tank when you can afford it. You'll be able to hide your heater and such in there. And maybe add some more rock to build up your scape - you can buy dry rock, which will only cost you a couple bucks.
Beyond that, it'll just be letting your tank grow and age. Maybe add easy corals here and there. Just keep it very simple and it could turn into a real beauty.