I would stick with EuroReef. An RS-135 would do the job nicely, and fit in your sump. The downward thrust of the water draining will make the detritus travel, so you might as well just drain the water into a corner bubble tower in the same section as the skimmer. You still can siphon it out.
The frag section will require a light over it, which may grow algae inside the skimmer. Cleaning it often will prevent it, or do a major soak (vinegar & water; muriatic acid & water) will get it back to brand new again.
I don't like external skimmers either as I used to have one that flooded my fish room way more times than I can count.
120g?? What happened to the 480g?
