how come something like a BCS walk behind with a bottom plow can dig out ground but a compact tractor can't?
The reality is, yes, you could open your field with a BCS or other heavy walk behind. It will take an awful long time, and you will be beat up, and the machine may well take a beating, but it could be done.
You could also spend the money to get the right tractor/tiller combo, and go after it that way. It can be done, but you need the right combination of parts. If you're in a rocky area, despite people saying it's fine, a tiller, and rocks isn't an ideal situation.
You can also spend the money, and go old school via plow, disc, and harrow to open it up. Not quite as nice as a finished bed compared to a tiller, but it's pretty darn bulletproof and simple.
Since you're starting at zero, you're looking at a steep buy-in to whatever way you go. Obviously you're going to need equipment, but it might be you do better hiring out the initial work. You could get the whole thing done to a seed bed the first year, and then work it in sections with a used walk behind as you rotate through different plantings. Give yourself a little time to actually figure out what you need, rather than just what might get recommended. Better yet if you've got some friendly neighbors that know more about your specific local conditions, where you might be able to see how they do it, and go from there.
There's no one right answer. You can throw a ton of labor at a problem, or you can throw money/technology at it. There's an investment to be made either way, and a cost to you, your wallet, and your equipment.