Two collections from Arolsen are already online and accessible from Ancestry:
Europe, Registration of Foreigners and German Persecutees, 1939-1947
This collection includes information gathered by the Allies at the end of the war concerning those who had been persecuted in Germany by public institutions, social securities and companies. The documents may contain death and burial information.
Africa, Asia and Europe, Passenger Lists of Displaced Persons, 1946-1971
The majority of the immigrants in this collection are Holocaust survivors or refugees leaving Germany and other European ports. These records were created by the International Refugee Organization (IRO) and the Intergovernmental Committee on European Migration.