When to buy breast pump?

Is it advisable to buy breast pump in advance (while you are still in second trimester)? I thought of using the year end promo sales to buy it. But one elder adviced against it saying it depends on how much milk I will be having. If too less then breast pump is not needed. How long will we be breastfeeding usually? Btw I'm not working currently. Will be getting back to work force after the baby is 5-6 months. So for SAHM is breast pump needed? Sorry if this sounds dumb. I'm a first trimer.