Skimmers take priority over lights. So between those choices you listed...get the skimmer first.
How about a good marine book?
If you are doing a reef, Natural Reef Aquariums by John Tullock is a good, very indepth book.
If you are doing fish only with or without live rock, The Conscientious Marine Aquarist also by John Tullock is another good indepth book.
Books with simple outline descriptions of fish, inverts, and corals along with capatibility and reef safe charting would include The Expert Pocket Guide Marine Fish by Scott W. Michael and the same series but on Marine Invertebrates by Ronald L.Shimek. Then there's Corals: A Quick Reference Guide by Julian Sprung. There are many others that may be just as good. These are just a few I know of off the top of my head. Another good series of ID books are the Baensch Marine Atlases.