It depends on how much of Hero Lab you want to end up with, down the road.
I personally went with the Core Rulebook, and the assorted Ultimate books which were available at the time.
I think it was Ultimate Combat, Ultimate Magic and Ultimate Equipment, plus Core.
So that wasn't exactly cheap, but then for a hobby that is fairly cheap compared to many others.
My goal was to have all of Hero Lab, picking up a piece here or there, as books were released and as my budget could manage.
Having all of the Hero Lab content for Pathfinder is really nice.
It you're looking for modifiers to initiative, you can type in 'init' in the feat section and the trait section, and so on.
Hero Lab essentially functions like a searchable database for whatever term you're interested in.
If you're playing a Monk/Fighter, and you know you're going to be unarmored due to the Monk levels (although you could be an armored Fighter/Monk too), you can search for Fighter Archetypes, which give up the Armor type class features of the Fighter... again make a level 1 fighter, and then in the archetypes area search for Armor Mastery or Armor Training.
That will get all of the archetypes that give up armor features, and one or more of those will get you a bonus that fits for your character.
You can also quickly check which archetypes stack with which others, since if one modifies or removes a feature, then another which modifies/replaces the same feature is invalid for that character; by taking the first archetype, anything that remains is still a valid option.
I use Hero Lab to check that player builds are 'legal', with everything stacking correctly.
While you may not need everything (for a given character), having everything dramatically improves the mechanical effectiveness of your builds.
I like to pick things which fit in with my roleplaying theme and the persona of the character, but to be as mechanically strong as I can go (without using broken combinations) from within the choices that thematically fit for my character/npc.