You should think more about how much light you want to use. If you have 40-50 micromols of PAR at the substrate, along with good CO2, and good fertilizing, you can grow whatever plants you want to grow. But, that light level is low enough that any algae that begin growing will do so at a slow enough rate that getting rid of it will be pretty easy. The plants will also grow at a slow enough rate that you may not need to prune them every week. If there is a reason for using more light it would only be to attempt to get as intense a red color as possible with those plants that become reddish only with lots of light and fast growth.
You can get 40-50 micromols of PAR with a single T5HO light bulb about 22-24 inches from the substrate. With an 18 inch front to back depth tank, one bulb will not give very uniform light from front to back, but two bulbs, widely spaced, 8 inches or so apart, will give you good uniform lighting, without exceeding 50 micromols by too much.
One way to do this is to contact Catalina Aquarium,
http://www.catalinaaquarium.com/store/index.php?cPath=71 and have them build you a pair of custom 36 inch long lights, using a wide housing made for 6-8 bulbs, to get the two bulbs widely spaced.
