Does a woman have a choice to work outside? The Prophet's wife Khadija(r) used to, so is it permissible?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is permitted for a woman to work inside or outside her house if the work place is free from prohibitions such as staying in seclusion with alien men, or mixing with opposite sex, or uncovering herself, etc.  For more details, please read the Fatwa: 82399.

As for what you mentioned about the wives of the Prophet (Sallallahu Alaihi wa Sallam) they used to go outside only for necessity.  The Prophet (Sallallahu Alaihi wa Sallam) said: You are allowed to go outside for your necessities. [Reported by  Imams Al Bukhari and Muslim]

Allah knows best.