Sun light is 5700Kelvin and up to 100 000 lumens per square meter. If I was to use Halogen or LED light's with similar properties would plants (sunflowers in particular) know the difference?
Lots of plants are grown under artificial lighting. Plants only need light to power photosynthesis so as long as you provide the correct wavelengths it doesn't really matter to them.
Sunflowers are going to be a problem for you because of their height. You would be better off growing a dwarf species, and using a reflective grow tent as they do when they grow marijuana illegally as otherwise you're going to need many more lights to give all the leaves good coverage.