I use all vegan substitutes... Well, not always for the white sugar, but for everything else most definitely.

White sugar - Turbinado. You can usually find this in the organic isle/section of your grocery store or in health food stores. You may also get by with using agave nectar, which is a great sweetener (although I've never tried it myself).

Milk - I use Silk a lot of the time, but I will also use almond milk, rice milk, or oat milk.

Butter - I almost exclusively use Earth Balance or Soy Garden. These are the best vegan butters on the market in my opinion.

You probably could use vegetable or canola oil. I haven't personally tried it myself, and I doubt that the calories would be any lighter with oil. Coconut oil might be an interesting choice as well.

These biscuits taste really great, so it should be interesting to find out how yours turn out.