i love seagarden plants, they are the only fake ones i buy. When getting them grab some tall ones for the background, and medium and small ones to create depth. As far as color, most of my plants are about the same color range (of green) and then used some red plants for the focal point (using the rule of thirds) which i placed behind driftwood, but you could use rocks or a cave. If you really want to get into it, you can play with shapes and techtures of leaves. Oh, and odd numbers work best: odd number of plants, odd number of type of plants.
how many plants really depends on how full you want your tank to look. I am sure that you can play around with the amount you want and add more along the way.