[TDerivan]

I know from personal experience that saltwater isn't any harder. It is definitely more expensive to get all of the equipment. You can get by with a good amount of live rock, some powerheads, and a good protein skimmer though. The water parameters will be different than freshwater, but certainly not more difficult. The best way I know to set up a saltwater tank (especially easy if you are only planning to keep inverts) is to do a deep sand bed. Starting with dry sand and dead rock will help to mitigate the usual startup pests (i.e. bristle worms, fire worms, cyanobacterial blooms, hair algae, etc.) and is cheaper than the live stuff. The only downside is that starting with dry sand and dead rock makes the cycling process take longer, but this can be sped along with something like Dr. Tim's One and Only or a similar product. I wish I had taken this route, it would have saved me quite a few headaches.
Macroalgaes make for a nice "planted saltwater" look. I have feather caulerpa algae in my sw tank.

 

[TDerivan]

I believe that the Heptacarpus shrimp are coldwater though and would probably require a chiller.
I could be wrong though.

 

[TDerivan]

The lighting would only be a cost issue if you plan to keep photosynthetic corals. Nothing else really depends on the light.
I would only do a 12 long if you've got a good sized sump for it. Most experienced SW folks usually don't do anything that small because the water parameters can change very quickly and have disastrous results. A larger volume helps to provide a buffer zone.