Wayner: any oil-based stain should work well on drycedar.
Bigfish: Composite or wood 5/4 decking need joists on 16" centers, or 12" centers for diagonal patterns. 2-by material (wood or composite) can be on 24" centers. If 5/4 decking is used on existing 24" centers, lighter joists can be used between the existing joists...if it now has 2x12, 2x8s between them would work and save some money. (5/4 (five-quarters) wood is about a full inch thick. 1-by (4/4)is about 3/4" thick; 2-by is about 1-1/2" thick.)
I'm just finishing a Home Depot "Veranda" brand composite deck. It rated well in "Consumer Reports," was lower cost than most other choices, and looks really good with the Phantom hidden fasteners. The fasteners are pricey and time consuming to use, but look good.