most importent with soil for weed, is to get some with in the right PH, a lot of cheap "super market" soil`s keep to much peetmoss and end up with a PH on 5,5 to 6,2
try to find one that is a bit higher, more like 6-7 (weed like PH around 6,2 - 6,8 as optimal)
if you choose the right soil from the start, and use the extra 2$ your have a more Smoot grow with less problems and more healthy plant`s
IMO it is one of the most important things, as its your plant`s home for +3 month`s
beside the right PH you should also look in to stuff that can make the soil less compact, weed like light and airy soil with good drain, stuff like Perlite, coco fibers and legs/hydro nuts/stones, green sand is all nice thing to mix in to your soil mix to keep it light
and I guess it go with out saying, the Pot`s need some good drain holes in the bottom
weed dont like to stand in wet soil all the time, it will use up the oxygen in the soil/water and end up as "over watered"