I'm curious, when is it actually mandatory to use the measure words like buah?
You can omit the classifier here.
Classifiers/counters are mostly used to make it more formal.
In the informal/spoken speech, 'biji' is also used as a classifier for fruit.
'biji' = seed, core of the fruit.
I would just say "ibu perlu dua apel" is this too informal or is it ok in speech?
In the informal daily speech it's okay to say it like that without using the affix.
This sentence is from 'me-kan' skill, that's why the 'me-kan' affix is used here.
Tell us please what is the rule for "me-kan" construction?
Should be "Ibu memerlukan dua biji epal "