You realize if you purchase a sack of grain you have to mill it. Right?
I am fortunate to have a LHBS in my town, so I always buy grains within a week of use and mill them at the store. If you review several recipies, you'll notice a pattern. Most grain bills start with *about* 2 pounds per gallon of base malt, then add up to 2 pounds each of one, two or three specialty grains. The base malt provides the enzymes to convert starch to sugar, and also provides most of the fermentables. The specialty grains provide color and alter the flavor. Since all recipies have a large proportion of base malt, you could stock up on that. Typical base malts in my neck of the woods are American 2 row, American 6 row, etc. Look on the malt analysis sheet for an enzyme content somewhere north of 150 or so to help you identify suitable base malts. You need an average of 60 points per pound in the total mash to ensure successful conversion. That means that highly kilned malts (with low diastic power) need base malt to supplement.