Don't pay for NAS! Get it for free (or super cheap). Here's what I did (and continue to do). Wait until someone throws out a computer. In my neighborhood, I seem to see one every 4 months or so. The latest one I got was a 400mhz Celeron(!). Before that, the best free PC I had was a 200mhz Pentium. This is more than enough to do what you need to do. Make sure it has a network card.
Buy a big hard drive. Install it in the PC. You will (probably) be better off having a small hard drive as the primary hard drive, and the big drive as the secondary, in case step 3 doesn't work out for you or you want to change Linux distributions.
Install Linux. On my 200mhz Pentium system, I installed Damn Small Linux (DSL) because it was small. On my "new" system, I am installing Ubuntu. The biggest negative with DSL was that I couldn't really use apt-get or whatever to upgrade the system as I wanted. Note for newbies: ignore everything I said. Just get Linux on the machine. It doesn't matter what distribution you get.
Install a ssh server. You can use Putty then to log in and change settings. My NAS server is sitting in the basement with no monitor, no keyboard, just a network cable and power.
Edit the /etc/samba/samba.conf file to your liking. There are much better tutorials than this. The items that I have that I use are a