Your questions are great! The partially composted leaves would make a perfect dressing around any plants, just like the forest floor. If I was going to buy anything, it would be mulch, finely shredded hardwood.
IMVHO, you don't buy good soil, you make it, over time, by adding organic matter to the surface. It's not hard at all, just use what you have, when you have it, the more variety the better, but not so much added at any one time/spot that it raises the ground level around trunks/stems. Mowing before grass makes seeds is another great source of organic matter, powerful nitrogen. Spread on the surface, grass clippings turn brown in a couple days, and decompose quickly. Kitchen scraps are another source, can be composted first, used "as mulch," slightly buried if icky. Whatever way you can use them is great.
You don't need to be a scientist to understand this lady talk about soil.
http://permaculturenews.org/20...
I'll leave the answering of the root ball raising questions to those who have done that before.
