You could buy 20 now, then buy 20 next week, and then the last 10 after another week. You don't have to get them at the same time.
If she is going with feeders, more like 20 this week, 20 next week, and 20 more in 2 weeks. Feeder quality isn't very high, and if I start a tank cycle with feeders I expect die off.
edited so folks may get what I am saying.