Unlike one in English, un cannot be used alone here.

En is a pronoun that refer to appartement :

...ou en louer un. (or rent one of them)

means

...ou louer un appartement.

Unlike one in English, un cannot be used alone here.

En is a pronoun that refer to appartement :

...ou en louer un. (or rent one of them)

means

...ou louer un appartement.

Similarly, we can't say:

Je veux un. (I want one)

but we say instead:

J'en veux un.