You can adjust this recipe to have even lower fat, or use all full-fat ingredients. I use regular sugar and one can of full-fat condensed milk, but you could use Splenda and use non-fat milk for both cans. You could also use egg substitute rather than whole eggs. However, I wouldn't go any lower than reduced-fat sour cream, the non-fat is too thin and runny for a topping. And do NOT attempt this with bottled lime juice - blech.
This was originally an Emeril recipe that I tweaked to be low-fat.