Whether or not the RAID configuration totally erases everyting is dependent on how you setup the RAID array.
When you are ready to add the second drive, start by backing up your data (as a matter of fact, you should backup your data on a regular basis) and then power the unit down, install the new drive and power it back up. When you login, the unit should prompt you to format the new disk, and there should be a checkbox to set up RAID1 - check the box, format the drive and restart when prompted. After the restart the unit should synch the disks.
The print server now.
Not all printers will work well with a print server - this is actually a function of the printer driver. The easiest way to get it working is to connect the printer directly to the computer and install the drivers and then, after test printing, disconnect the printer from the computer, connect it to the DNS-323 and then change the printer port.
Add a new local port and call it \\<ip-address-of DNS-323>\lp.