All depends on what you plan to do with the NAS and how much space you will need in the next X number of years that the NAS is online - whether just for pure storage or if you plan to add apps for torrenting, downloading, web page serving, media serving etc. Just note that if you torrent 24/7 on a desktop, your electric bill is gonna rise. My advise - keep torrenting on a separate low-powered Linux-embedded NAS like Dlink or Synology etc and keep the NAS powered on when you need it.

The 500GB HDD seems minuscule, if you plan to host media files on the box. FYI MP3/FLAC albums runs 150-250MB each and 1080P movies runs around 5-9GBs each. Also if planning for RAID 1 redundancy (easiest to setup and fastest to replicate) you will need a free SATA port and a HDD bay on the box. I'm not familiar with Lenovo Thinkcentre Edge 72 (SFF model). How many 3.5"/2.5" drive bays, SATA ports and PCIe slots does it have? Seems like a small box. Depending on the current mobo spec and what you want, there are ways to expand the storage space.

On the software side, you will do better without NTFS for your NAS. Linux file systems are more efficient. 2GB RAM is a great place to start with any Linux distro. But if you are looking into ZFS or Win Storage Svr 2012, you will need more RAM.

2GB ram is more than enough for a linux system. Remember it's not a desktop. It is a server so no need to install the linux desktop environment if you don't need it. I used to run my ubuntu with 1GB ram and it is totally sufficient for almost everything I throw at it. I also install XFCE which is a very light desktop envinronment with minimal stuffs.

With 1GB ram, I was running deluge torrent, serviio DLNA server, apache (for serviio gui), samba file sharing, software RAID, a few java apps, vnc server (with XFCE). Until I went and throw in a camera monitoring system. This prove to be slightly overwhelming for my linux box. however, it still can run fine all the apps with some load only. That is even due to partially processor not power enough.