It all depends on what you want to do with the iPads and how you will use them.
You must consider connectivity, storage, personal usage of crew etc.
Corporate purchase and governance may be an option.
There are some basic documents which refer to EFB capability, types, classes and so forth that should be read first as a starting point. Search the FAA website for EFB and the circulars will show up. (AC No: 120-76C is a good starting point)
Regarding devices being able to be used, most iPads can be as they are already proven to be effective and decompression tested etc, however you may need to obtain the certificate for that to prove you looked into that or have done the test yourself (expensive).
FAA may require you to be able to exert control to ensure version control of documents, corporate guidance to ensure document validity is maintained during use and so forth...