I think if you're pretty sure you're going to be using one later on at least, then it may make sense to buy one, yeah.
Member rental rates are $15 or $25 per week, depending on the pump. Which is fine if you only use it for a couple of weeks!

ETA - if you look on the Motehr's Direct site, they give some info on who each pump might be suitable for, which is a good starting point for comparing pumps.
Maybe wait for a sale to buy? you have plenty of time at this point anyway