HDD For Media Streaming?

If you want RAID then you should match your drives. This means either an additional 1TB WD Blue or a pair of matched drives (any capacity and model).

Personally, I think NAS drives are exactly what you want. NAS drives are designed for 24/7 operation and you said that the server would likely be running all day. WD Red seems like the obvious go-to here. Particularly where the data is being sent over a network, the network is going to be slower than any HDD or any RAID array. Even if you have 1Gbps hardwired connections you're unable to get any more than 120MB/s and a WD Red can definitely perform at that level. Plus streaming video is rarely more than 30Mbps (about 4MB/s), so even if you have two people streaming from your server you only need 8-10MB/s of bandwidth.

IMO, WD Blue are cheap drives meant for cheap builds by people who consider storage an after thought. WD Black is a high performance drive suitable for 24/7 use but expensive. WD Red is meant for 24/7 use with lower performance, relative to Black, but is excellent for storing large files and in NAS applications (where streaming is a specific application of a NAS environment). WD Red Pro is more expensive than Red and meant for NAS with more than 4 drives since it includes some anti-vibration stuff that can arise when you have multiple spinning drives nearby. WD Green is essentially low-power archival stuff, suitable for external drives that are turned on basically as-needed and not for 24/7 use - they're also commonly seen internally as storage drives that don't really get accessed often and aren't good for, say, an OS or storing games you're actually playing. WD Purple is marketed towards surveillance applications so it's best for 24/7 use with heavy write demands. Since you're streaming, that's heavy read demand with light write so Purple isn't the right choice either.

For your needs, I'd actually suggest 5400RPM since it has enough performance and will generate less heat and use less energy than 7200RPM. So, Red is better than Black here. Cache size... more is better, always. The cache memory is always faster than the drive itself, but realistically what this means is that stopping and backing up isn't too painful.

Personally I would say if you're serious about doing this correctly then you should get a RAID controller with RAID5 online capacity expansion and online RAID level management and three matched WD Red drives in the 2-4TB range. This would give you 4-8TB with the ability to add extra drives later for an extra 2-4TB or to convert to RAID6.