
Itchy |

It Depends. What books are you using? Buy the packages that include those.
If you are only using one or two options out of a book, you could get away with learning how to create those options on your own in the editor (trade time for money).
Are you the GM? Or are you playing?
If you are playing, I would recommend:
-Advanced Player's Guide
-Ultimate Equipment
Less strong recommend, but not necessary (unless you are using options from these):
-Ultimate Magic (if you are using it)
-Ultimate Combat (if you are using it)
If you are the GM, then you need to buy everything that your players are using. You'll also NEED all the Bestiaries (The Bestiary bundle).
I would also strongly recommend looking for Shadow Chemosh's added content (Shadow Chemosh is a Hero Lab Guru) and the community bestiary.
I'm sure much more qualified folks will have more and better advice.

Bilbo Bang-Bang |

I have been running core set, APG,UC for sometime now. Like since 2010. If you need something specific, you can generally use the writer to create it from other sources. But, overall anything you buy you will not be disappointed by. The only exception is books that are heavy on fluff and not on crunch. Advanced Races is also worth an early look for extra crunch value.