When you buy the building your lawyer should have it written in the contract that all idols and religious items should be removed prior to sale. Prior to closing you should inspect the building to make sure there are no religious objects.
All that is needed is the nullification by a non Jew, of the church. So for example if a non Jewish employee would crack or dent any idol or cross or other religious object prior to taking possession there should be no problem.
Please contact a Posek before purchasing a house of worship as any non Jewish religious object must be destroyed and you can’t get a benefit from it once the Jew takes possession of the building.