Make sure they're all quoting on the same thing--a condo (the association, as Gerard notes, will have its own policy), not a single-family home or a rental unit. That may account for the price variation.
As Gerard says, also, find a reputable agent. Also, preferably, go with a company you've heard of and recognize. Stay away from Internet-only type companies. (They might be good . . . but they might not be.)
Hope that helps.
Any policy you purchase will cover the interior of the unit.
Find a reputable local insurance agent to advice you - and if in doubt regarding the coverage - ask them to walk the unit with you.
A good agent will do this!