My girlfriend is going to be taking a new job that's going to turn her 2 mile round trip commute into a 26 mile round trip commute. Her bike is currently built up as a single speed, and she thinks with the longer commute that she definitely needs to get some gears! I was thinking that a 1 x setup would work because the commute is flat, so the gearing is for the couple of over pass hills, the wind, and tired legs. The problem is that I have no idea what speed/gearing cassette to even start looking at for a 1x setup, any recommendations?

I should also add the current front chain ring is a 46t, though if I had to change it no big deal.

Thanks

What does she currently have in the back? Is she more or less happy with it? Does she ever spin out? Are hills difficult at times?

Assuming she's pretty happy with the current gear inches, I'd start with that and make sure she's got options for going both higher and lower. On the other hand if she rarely spins out as it is, she's not going to need much more on the higher end than that what's already there so maybe you concentrate on getting her more options on the lower side. Those are the things I'd consider.

Internal gear hubs have a wide range of gear ratios see the range options..

https://sheldonbrown.com/gears/internal.html

i like my Rohloff hub , 14 ratios 13% apart, but they are German precision engineering , and made there . (so not cheap)

Shimano has a couple [Nexus/Alfine] 8 speeds , and an 11 speed waiting in the wings..

For my winter road bike I run a 42 t up front and a 28 -12 8 speed out back. A 28-12 either 8 or 9 speed should work well with the 46 t chainring.