The roses will dry in 3 to 5 weeks. The sand does not absorb the roses moisture, but holds the roses shapes while they dry naturally. There's a spray preservative called Victoria's available at many dried flower shops that strengthens the rose and preserves the color. Nothing worse than having a great rose fade to brown. Also, many shops carry liquid or spray dyes for roses which can produce great effects.
Floral wire is the fastener of choice, and is very easy to work with.
You should not use preserved roses. Paint them while they are still moist. The hairspray is the special secret trick. When they feel dry enough, you can completely drown them in the stuff. Just check they don't go mouldy.