If anyone has any suggestions, edits, vague ideas, or revamps feel free to let us know.
About what you have asked...I can say that the embark may take a little while to dig out, but a solid group of miners could finish the job with relative ease. Just know that if you were to dig out those layers, little time would exist when you finish for you to build a wall or some sort of shelter before you start getting harrassed by goblins and kobolds. I would wait until you had a sizable migrant wave before you draft some miners, because from the start, you will need masons and/or carpenters (whichever applies to what materials you want to use) to make your fortress defensible.
Also, I don't think adventure mode dwarves mind drinking water, such as long as they don't get thirsty.
As for vague ideas, I have plenty I could share that I find makes surface-dwelling an enjoyable experience!
If you like variety, and plan on creating a town surrounding the bastion, try and make buildings set to specific purposes you can relate to. For example, create a bartender profession, and assign a single dwarf to it. I find that building a pub (complete with a dining hall) is much more flavorful than just assigning him/her to a still to just churn out alcohol. For food, build a restaurant, and for berthing, houses and elaborate apartment blocks. I find the game to be infinitely more enjoyable when I do not build symmetrically, or even too efficiently.
If two dwarves were to marry, try giving them a house to live in. In doing so, marriage becomes a memorable occasion in place of what seemed like just flavor text and impending children. The party following the ceremony might happen at the pub, or the restaurant, or possibly even in the newly assigned dining room of the house that was gifted to them! I have always remembered these parties over those in fortresses containing all-purpose meeting halls for everyone that is not nobility.
These fun little details come easily to me only for surface fortresses, for the simple reason that I can relate to them. Other reasons exist, and I could go on and on about things I love to do in these fortresses, but this post could go on forever that way. Good luck with the bastion--it sounds very ambitious!