What additional lights can be added

Any LED lights should be alright and as much as you want, as long as the total wattage used by your set is not more than 40W. The total wattage of your set is the sum of the wattage for each lights you have in your set. This kit already includes 1.8W, so there is still 38.2W left for you to add some lights. I would recommend you to use LED lights since they use way less Watts than incandescent and halogen lights, but are still as bright as them. There are many sold at Lowe's online.