To answer your question a little more defintivately, yes the Motif is most definately a sampler. It's not just a dedicated sampler but its got all the functions.
In order to sample you need RAM (this is the area the samples are held). The old Motif comes with 16megs expandable to I think 64, the new ES comes with nothing and is expandable to 1gig, although they only advertise 512. (2x256 or 2x512)
The reason you would need a HD or USB hardrive, Smartmedia etc is to back it up because RAM dissapears when you turn off the system.
The ES does not have SCSI support.