Weeeeellllll.... I'm just being honest when I say this material is going to take a lot of work and a lot of time. You may find that your patience with the material wears out before the tree itself starts to look good. But that's ok! It is best to learn, and then look back and say "NOW I know EXACTLY what they are talking about." We have ALL been there, and learned the lesson of how much to expect from a tree... The other lesson to learn - it is best to fix problems NOW and give the tree a good start EVEN if it will take years to grow to potential, than to try to short-cut the process and say "well I don't really want to chop it too hard because it will take too long" and end up with a tree that will NEVER be worth anything much.
Right now this tree has two assets: its trunk flare at the base, and its roots. Anything more than 6" above the ground isn't really worth salvaging in my opinion. If you were to do an airlayer up high, you would be left with a smaller scar, but a smaller trunk as well.
So why do I say the roots are an asset? At this point you want powerful growth above all else. Powerful growth means more back-budding, faster growth of shoots, and faster healing of the trunk scar. DON'T TOUCH THE ROOTS (at this time)! (i.e. don't try to repot it, or trim the roots or anything). Putting the tree in the ground provides even more power for the tree to grow fast and strong. You want the tree to be as powerful as possible... and then you can see for yourself how quickly the scar heals, what the trunk looks like as the scar is healing, whether or not you cut at the right spot, whether the buds pop in the right places, etc.
There is a little poem I remember any time I transplant a tree:
The first year they sleep.
The second year they creep.
The third year they leap.
Put the tree in the ground and fertilize it like crazy and the trunk scar MIGHT heal in three-five years. It might take longer, but it certainly won't be shorter. Don't put it in the ground, don't fertilize it heavily, don't let the shoots grow strong, and the scar may be there indefinitely. This is how much pre-bonsai material is developed - it is field grown and chopped, and grown, and chopped. Each time you chop it a little sooner, and don't let the shoots get quite so big, and each time the scars are smaller and the trunk taper is better. I am leaving out a lot of nuances, but that is the general drift...