To underline Adam's point, the DA-L 55-300 has the same optics as the DA and DA WR versions (the WR version has newer coatings), for considerably less money. The plastic mount is more durable than it looks. QuickShift and WR are nice to have, but aren't related to image quality. Buy a generic hood for $5 and you are set.
For about $US120-160, it's one of the best value buys around. It can produce very good images.
Use it for a while and enjoy it. If you decide to upgrade to the PLM 55-300, you won't lose much on selling the DA-L.
As for the Sigma 50-500, it's known as the Bigma for good reason. A 2kg lens is a beast to carry and use. If you haven't shot much telephoto before, the 55-300 is a good lens to start practising with. Works nicely with the K-S2 as a walk-around combination. Even if you end up buying something with more reach, you will probably still want to keep a 55-300 as a lightweight and portable option.
Technique matters a lot in shooting with long lenses. See this very helpful article:
Making the Most of Long Exposure Handhelds - Introduction - In-Depth Articles (It's about long exposures, but it's really about steadiness generally.)