Unless you are encoding significantly faster then real-time, or, parallel encoding, RAID could be considered useless. In a fairly tight budget situation, don't even bother.
SSDs are bundles of joy, because their access times are near instant. HDDs have access times around 13ms. SSDs are in the neighborhood of 0.1ms.
SSDs work extremely well as OS drives, because the OS is constantly reading (and to a lesser extent, writing), significant numbers of files. And instead of having a 13ms delay between each file read/write, it all becomes basically instant.
Of course, it also helps that SSDs have excellent throughput. In the neighborhood of 500MB/s, vs 130MB/s for a decent HDD.
