I would stay away from buying a new AquaC Urchin or Remora / Remora Pro, as they just don't perform well enough for the cost. I like the Remora and Remora Pro I have, but I wouldn't buy either one new (I got both of mine used).
The AquaC EV-180 skimmer I have though, now that's a different story. I'm not saying that it's the best skimmer for the money (there's just so many choices), but it vastly outperforms the Remora / Remora Pro skimmers on my tanks. I would definitely consider buying another one if I needed it, but again, there are many other choices that probably work as well or somewhat better for the money. I think the EV-120 might be big for a 48g tank, unless you plan on stocking heavily or are striving for very low organic levels for a strictly SPS tank.
A 20g long won't be hard to turn into a sump at all. Any of the skimmers you listed (and many others) would be easy to fit into that size sump without a problem. When making your sump, it is generally a good idea to maximize the area for the return pump, as that is where the water level drops due to evaporation. Of course, it ends up being a balancing act, since having a large refugium area also has its benefits.