Thank you for the voice of reason--if you don't know, don't start until you do as you effectively ruin the market for legit firms who price work based on what it costs + markup for profit.
Replacing of soil is almost always a no-no and certainly amending with some unknown topsoil can also be a recipe for disaster. Not knowing what plants will work doesn't instill confidence either
Equipment issues aside the three things above are bigger concerns--we should all be striving for excellence at being craftsmen in this trade--this is a skilled trade and commands the premium wages, but you have to do your time as an apprentice and learn the trade. Pricing is a key part of this, knowing the " whys, hows, whens, ifs, ands, and buts" is crucial to delivering a project that has long term success and generates a happy customer and profit in your wallet.