I cook mostly dairy free and it is very easy to substitute for milk.  I have tried various "nondairy milks, " but I find the best substitution is coconut milk.  However, you may substitute straight across with any "milk" that you already use.  I buy lite coconut milk in a can and mix it with water for my milk substitution.  I find that you don't need very much to make a great product and there is no coconutty flavor.  I use about 3 T. lite coconut milk + remaining water to equal 1 cup.  The great thing is you don't have to be exact, but I find this is all you need.  Sometimes I use more and sometimes I use less.  Once I open a can and use what I need, I pour the extra in a container and keep it in the fridge.  We usually use it within the week.  If you buy full fat coconut milk, use even less, probably about 2 T. per cup of milk.
Use the coconut milk water ratio + 1 T. lemon juice per cup.  For example, to make 1 c. buttermilk, grab a liquid measuring cup and place in 3 T. lite coconut milk, 1 T. lemon juice and water to make 1 c.  Let sit.
I always use the EarthBalance dairyfree(and sometimes soyfree) alternative.  It cooks great.  I will also substitute butter with Spectrum shortening depending if I need the butter flavor or not.
There are many flours, all with varying tastes and qualities.  In the beginning I tried them all and I picked the ones with the tastes we liked best.  I use sorghum, millet, and teff the most.  Sorghum and millet are more bland and I use them to create a "white flour" taste.  Teff has a stronger taste and is great in anything with chocolate or anything that doesn't depend on a bland flavor, ie. sugar cookies.  Feel free to substitute any flours you use with the same quantity.
My son doesn't tolerate corn starch so you won't find any in my recipes.  You will mostly find tapioca starch and sometimes potato starch.   You can substitute corn starch for potato starch, or arrowroot for tapioca.
Coconut Flour
Despite my hatred for "coconut" I couldn't live without it.  I figured I just don't like the fake taste of coconut or coconut flavor in sweet things.  I cook daily with coconut milk and coconut flour and love them both.  Coconut flour is tricky and cannot be subbed straight across. It is full of fiber and soaks up a lot of liquid.  I started using coconut flour to cut back on the starch and still achieve a great product.  Coconut flour creates a soft, moist product and is best in cakes, muffins, and sweet breads.  Because it soaks up so much liquid I use only a small amount.  For example, to replace 1/2 c. starch, I will use 3-4 T. coconut flour and sometimes add 3-4 T. water.  Double or triple the amount of coconut flour to achieve the amount you are replacing.  If you are not cooking with coconut flour, to achieve the same light, great taste of my recipes, 2 or 3 times the amount of coconut flour and replace it with starch.  It is well worth buying coconut flour - it has no carbs and is full of nutrition.  It goes a long way because you don't use very much.  It has no gritty taste and is a dream to cook with.