To clarify, is the property a REO, or would you actually try to pick it up at the foreclosure auction.
You have pretty much figured it out, the Protecting Tenants act gives the existing tenants 90 days. You would just need to be prepared to handle an eviction if the tenants were recalcitrant.
As the others have stated, you will want to run the specifics by a real estate/land lord attorney to cover your bases.
With all of your very good questions surrounding the property an the transaction you would be well advised to consult with a real estate attorney who could properly advise you in greater detail and with more reliability than this forum can provide.
You really do need to sit down with a real estate attorney to cover all the bases.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.