What stiffness for beginner taking lessons?

I’m starting some lessons soon.

I’ve been to the range a few times, 7 iron speed club speed fluctuates between 70-90mph.

I’m starting a block of lessons and just want to know which shafts I should look at?

I’m probably slower swinging when I’m actually on course but that might speed up with lessons?

I’ll be buying a secondhand cheap set so just want to know which to look out for. Any help appreciated!

For additional I’m just using a friend’s set at the moment, they’re regular shafts.