I think you have two approaches:
1. Create a fully stocked bar with all the bitters, mixers, etc.
2. Work backwards from 3 "house" cocktails and just buy the stuff to make that.
I'd suggest starting with option (2) and then just add to your repertoire of cocktails rather than starting with the liquor side of the equation. Yes, you will end up with things that are just used for a single cocktail, but at least you will know you like that drink.