I don't think there is an advantage either buying or building. I just depends on what you got more of: time or money. I don't always have much time, so if I can buy something I know will work, then I can figure out what I can get done when I do have a chance to turn wrenches.
Now for something completely different... how about paddle shifters? If you are going to use a 4L60E or 80E, it might be an option. Dare to be different!
