Yes, AFAIK this is correct - you can always expand the internal storage of the 212J later by adding a second hard drive (and presumably converting the volume to RAID in the process).
Note however that any external drives can't be part of a RAID array in any way, so they will always be standalone volumes (one for each external drive), available from the NAS as usbshare1, usbshare2 etc.
DS1812+ 4 x 3 TB Seagate Barracuda (SHR), 2 x 2 TB Western Digital (RAID 1), DSM 4.0
27" iMac 16 GB RAM, 3 TB Fusion Drive, NVidia GTX 680 2 GB, OS X 10.8 Mountain Lion
Mac Pro 6 GB RAM, 120 GB Intel SSD, 1.5 TB Western Digital, ATI Radeon 5770 1 GB, OS X 10.8 Mountain Lion Server
Mac mini 8 GB RAM, 2 x 500 GB HDD, OS X 10.8 Mountain Lion Server