Have you considered each of the Six Honest Serving Men?
What, Why When, How Where and Who?
Here are some more questions in no particular order -
How long do you expect to stay in this house?
How much are you prepared to/able to spend?
Will you get professional help?
What will be your time scale for the design?
Will you phase it over a period?
What will be your order of priorities?
Which views will you focus on or screen?
If you have lost any plants over the last season or two do you know why?
Have you done a soil test? (Inexpensive kit is fine.)
Do you know where north is?
Will you draw a plan to scale? Even a rough sketch is helpful.
Do you have someone who will water while you are away?
Lots of practicalities and perhaps the illusion of control....but maybe what we are after in a garden is the daily and intimate contact with the miracle of nature....right on our doorstep, the newly opened flower, the bee, the butterfly, the scents and colour - a garden is a sensual experience, and should have an element of enchantment and joy.