"We own an apple" should be correct too. In Greek "έχω" may mean simply I have, but also it carries the meaning of "I own".
Isn't to own though a lot more formal and closer to 'ανήκω΄?
ανήκω is "belong", I think, not "own" -- αυτό το σπίτι ανήκει σ' εμένα "this house belongs to me".
Maybe κατέχω for "own, possess"?
Μας ανήκει ένα μήλο is what I had in mind, but I find it too formal - for an apple no less!
Κατέχω does not sit well with me either, because of its associated meanings, it's too possessive.
Yes, I'd use "own" for property or something of greater value than an "apple".