IMO- yes get a skimmer, you'll need as much water as it takes to fill up your tank. If you don't get the skimmer, youll need to do more frequent water changes. I do w/cs every two weeks. Live rock is a big part of biological filtration in the tank, so you can get by with less rock, but it's still a good idea to get some, I'd say at least 20 lbs. get dry rock first, then add a piece or two of live rock. I would just get at least one powerhead, they will look much nicer.
Look up caribsea Fiji pink sand, aragonite sand helps buffer you ph, and looks good too.
A sump isn't necessary but if possible do it. Eventually you will want one.
That stock looks ok, although I don't believe the blenny will burrow. Look up watchman gobies.
The key to starting a sw tank, is to do it right. If you don't have enough money to get the right equipment, wait until you do, because you could end up with a frustrating mess. Also read greech's sticky on starting a new tank.