Eosin the Red said:
You only need the Hero System 5th Revised but getting a villains book and the champions book would go a long way towards easing the GM burden.
And that's what it really comes down to. How much work do you want to do yourself? What do you want to have done for you? What do you want to pull ideas from? And how vast a scope to you build campaigns before they start?
You definitely only need the core book, but at various points you may want to consider:
Champions 5e - to help with some GM tips for supers, and using the rules to best effect in a supers game. It also has templates (for lack of a better word) for supers archetypes. If you get only one supplement, get this one.
Champions Universe - If you want a setting that's out of the book (and the one all the other setting books reference). They have two city setting books as well, if you want your setting to be a city rather than an entire supers continent, planet, galaxy, etc.
Until Super Powers Database 1 and 2 'if' you want to save time calculating out typical power builds.
One or more villain books if you want some ready made villains, or want to recycle the builds into your own villain ideas, use it for inspiration, etc.
Ultimately, you can do everything with the single book. The rest are designed for simplifying the GM workload, and/or setting material.
For a more streamlined version of the rules, you can get Hero Sidekick. Or even an old version of 4e.