How I would do is, A-Z; 1-0 (2 sets + extra 2x1 0) with numbers only; punctuation; F1-F12; nav and cursor; 5 extra 1x1 blanks; ctrl/alt in 1, 1.25, and 1.5; win/menu in 1 and 1.25; shifts in 1.25, 1.75, 2.25, 2.75; ANSI and ISO enter, caps lock 1.75 (x2 center and left stem mount); tab and \| in 1.5, backspace 2x1, enter and + in 1x2; spacebar in 6, 6.25 and 7.
Should be good for almost any physical layout, and fairly universal for ANSI and most ISO layouts. Since DSA is flat, being able to move keys around to the different locations is easy so I think it would be silly not to include ISO as you could get some good interest there.
I would just make all of that 1 set, no options. I hate all the add on kits it just make things more complicated and more expensive. If you just have all those keys as default it brings the prices down as there are more keys so more volume automatically. Since SP give volume discount it can make the sets cheaper overall in some instances.
Also many people end up switcing boards and could end up needing some of those keys later, or if selling having those keys could be the difference of selling or not in some cases. Just some thoughts.