You can buy cheap LED's and you can buy expensive ones, but if they aren't the correct wavelength, they are a waste of time and money no matter what they cost. The light Harvey mentions is virtually useless for plants, unless the "Specifically Pick Red Wavelength is > 630nm*" and "Specifically Pick Blue Wavelength is <460nm*" gibberish means you can specify LED's at 660nm and 420-430nm, which you can't. This is the problem with 99% of the LED "plant lights" sold - they are simply the wrong wavelengths. Sure, if you focus a 100w worth on a 3cm point, you can get something to grow...
Re-read the post from thepaan with the chlorophyll curves. The importance cannot be stressed enough. Plants prefer chlorophyll a and only use b when they aren't getting the a wavelengths. There are always exceptions, but this is generally true for all plants. If you're going to go through this much effort to work out the electronics, make sure you're not wasting your time - the purpose of the project is to grow plants.