Used quality equipment IMHO.
Also, patience... so much of what we pay huge $ for can be had for free, or nearly free, if you're in no hurry to get it.
Watch local FB groups. People often are shutting down a tank and just want a home for their fish.. maybe a small "adoption fee." Silly little things like the red sea titration handle. The test kit is $50, but the refill is $20. Use two hands, or be on the lookout for one of those little handles in someone pile of "fish stuff, has to go today."
A local reefer friend can save u a ton too. Why both have 7 different medicines on hand that will expire before they're used? DIY and corrugated plastic are your friends
If you end up with a more advanced tank, RandyHolmesFarley's DIY section can save you 70% easy on dosing stuff... vinegar and sugar instead of brand name carbon dosing... Nitrate and Phosphate dosing solutions that will make gallons of additives for the price of a 12oz bottle of additive online.
The big expenses jump up when you've screwed up and are faced with spending x amount of money to try to rectify a mistake or watching all you've already spent die. Spending some up front on good testing and good water source WILL save you way more down the line.
System design too.. There are so many ways to skin a cat. Like Lynyrd Skynyrd said, "Run a Simple Kind of Tank."