Welcome to the NL world
If you need to install on a HDD, you need NL 2 HDD
The HDD that will host the NL OS will be mainly used for NL. It is better to have à USB version with a small USB key that holds everything (OS and the settings). If not, many have a CF card with a card reader to install the HDD version (I haven't used this option thus I only know what I have read on the forum).
NASLite v2 is better and faster. All 3 versions (CD, USB and HDD) are the same, only the place where you install it changes. CD version needs also a floppy to save the config.