If you need the skimmer now, get the Octo NW 200 ($200). If you can wait, get the MSX160 ($310). The MSX200 is $360 and while it would work fine, it may be overkill for your tank.
As I mentioned before, the ER RS180 is basically the same as the NW200. They have almost the exact same dimensions and same pump volume. Personally, I would buy a new skimmer that costs less before getting a used one that costs more.
Mods are done to improve skimmer performance. I did quite a few mods to my Octo. The easiest and cheapest mod is putting mesh on the impeller. I bought the parts to do the gate valve mod, but haven't done it yet. All that does is lets you adjust the water height in the skimmer without getting your hands wet. It does nothing to improve performance.