I recommend looking at the manual to get an idea of features -->
ftp://ftp2.dlink.com/PRODUCTS/DNS-320L/REVA/DNS-320L_REVA_MANUAL_1.30_EN_US.PDFAlso D-Link Canada has some FAQs regarding your questions (use internet explorer) -
http://support.dlink.ca/ProductInfo.aspx?m=DNS-320L - 2nd page
To answer your questions:
1 and 2 - Do a hard reset (paperclip in the reset hole for 10 seconds). This will unregister the NAS with his account. Once you register it, it is yours. Even though he may have the mydlink ID, he cannot over-ride it and put it on his account or view any information from your account.
TBH I am no expert on NAS units so if other forum users can give better answers or add-ons that will help...
3 - yes for the most part. You can create groups which you give permissions to folders on the NAS, then create users and assigned them to the group. For access from the Internet it is best to configure the NAS as a ftp server.
4 - yes. You can map a drive to folders on the NAS and use internet explorer.
5 - You can use the mydlink lite app or mydlink access-NAS app for android.