If you are looking for models that are different then what you have already then you better get the add ons discs. You will not beable to download any models or variations of models unless you have the add ons disc that has the original model on it.

Say you want a 540 or a piper cub and someone has a customized 540 or Cub for download. You will need the add ons disc that has these models first before you can download the customized ones.