I have my old i5-750 CPU, motherboard, and 4GB RAM sitting around so I bought a power supply and 8x3.5" bay case to build a HTPC plus storage box. The motherboard is an ASUS P7P55D-E PRO which has a 6-port Intel controller and a 2-port Marvell controller.

I want to run Windows because it's what I'm comfortable with and it'll allow me to run all of the applications I need.

However I'm not sure the best way to set up my drives -- either 6 drives in RAID on the Intel controller and a single OS drive, or two OS drives in RAID 1 on the Intel controller and the storage drives spread across the two controllers and using Windows Storage Spaces to make a parity storage space.

I have 4 old WD 2TB Greens that are way past their safe cycle count that I've been using to test setups. I also have two 640GB WD Blacks laying around and I'll be using one or both of those for the OS.

For my real build, I plan on using 6TB WD Reds as they aren't much more $/TB than smaller drives and it'll make better use of my physical space. In an ideal world I would start off with 3 drives and add more to the RAID as I run out of space but that doesn't seem possible with the Intel controller and Storage Spaces doesn't seem to redo the parity when I go from 3 drives to 4 (the storage usage doesn't seem to get spread to the 4th drive).

Also I'm not sure if one of my old drives is extra garbage or not but Windows Storage Spaces write speed seems extremely slow, and RAID 5 not much better. Both are less than 20 MB/sec. Is this normal?

I'm also not sure which version of Windows I should buy (8 or Server 2012). I'm just reusing my existing copy of Windows 8.1 for the moment during testing.

Any suggestions or input on my plan? I guess the slow speeds could be attributed to the garbage Green drives -- should I buy 3 Reds now and see if it improves? Or should I go to the immense expense of a RAID card?

This is just a Plex server and network storage box -- I won't be doing heavy read/writes other than the burst of allocating out space for a new torrent download.