LEDs and autos are a good fit. Where most of the 3 watt panels available today don't have sufficient penetration for growing photos, they are perfect for autos. When you compare their useful life compared to HPS, factor in the savings on electricity and replacement bulbs, they come out cheaper to run that HPS over the long haul. But since you're already got the HPS, why invest in something else unless you're really intent on making a change.
Regarding the nutrients, you will still need both veg and bloom. I assume you are growing in soil. Depending on your soil mixture you may need veg nutes starting anywhere from week 2, 3 or 4, through week 6, then flowering nutes after that. Autos are a lot more nutrient sensitive than photos. Always start with a low dose, usually 1/4 strength, and work up. I've personally used Fox Farms, General Organics and Advanced Nutrients with good success.