But all of this stuff is really secondary to what you really need, inspiration. The tools won't be any good unless you have something to say.
I agree. I would think it would be easier logistically and economically to start with a more basic setup and makes additions to it incrementally. Otherwise, it may not always be clear what each component can add to the mix and in what order in the chain. Of course, it helps greatly to have some music training to begin with and know some basics about synthesis, too. The less prior training you have, the harder it might be for you to carry out what you are hearing in your head.
I want to start differently. When I started in digital arts, I was always frustrated because I didn't had the money and I couldn't afford the tech tools I wanted. It would be easier to start with the right tools and just be creative instead of fighting with faulty hardware...
I never wanted to try to mimic something that was in my head, I prefer to be surprised and be like a driver, I use that method in other areas and honestly I prefer it that way.
There's no "right" way to create ambient music, so there's nothing wrong with starting big, if that works for you. If by "right tools" you mean to include synthesizers and other tone generators, there are plenty of potential "right" choices. But I would resist the temptation to discard your old "tools" for sake of acquiring new ones if you haven't exhausted the possibilities of what you already have.
By referring to what you hear on your head, I didn't mean to suggest that you should try to hear the piece in your head first. I haven't worked that way in a long time. I am referring more the tendency to assume that music training doesn't matter in creating ambient. It really does matter in most cases. At a minimum, it should help you in structuring your pieces and in identifying different relationships within a piece.