In the past few years several West Coast guitar makers have been using cedar on steel string guitars with excellent results. Some guitar makers are also now using redwood tops. All in all, there is a tremendous amount of luck involved. If in doubt, dig in your fingernail. Luan will dent like a piece of pine or fir.
Real mahogany is fine for necks.
For finishing materials, I feel the best source is H. Behlen and Bro. They are a complete source for wood finishing materials. Virtually every guitar maker I know uses lacquer for finishing, and Behlen has a fine lacquer for musical instruments. Having tried two other lacquer systems, I highly recommend this one. It brushes nicely, is perfectly acceptable for spraying, appears quite flexible, is less likely than most lacquers to asphyxiate you while you're using it, and although it is soft at first, it dries to a hard finish.