I have both sellier and retourné K28s and in fact my last SO was originally exactly your specs except I had brossé PHW.
Some points: Chèvre Mysore is indeed available (only) for special order in a Kelly and you can get sellier in ANY leather, although Paris may refuse Clemence or Swift depending on their mood

Chèvre is a great leather although it starts out a little shiny IMO.
In a sellier your contrast stitching will be visible along every seam of the bag, as opposed to a retourné, where it will only be visible on sangles, flap, and handle. Consider this, as it will change the versatility of your bag. It changes the whole look of a sellier unless the difference is very subtle. My Ks with contrast stitch are either sporty (gold w/ white) or the contrast is subtle (fauve with Rubis).
That said, it's a great choice overall, although personally I don't like brushed gold with BE.
I changed my order in the end - to a 32 because all my Ks are 28 and I needed more space, to a Rouge H interior because I want a BE exterior bag soon, and to Togo sellier, partially for studio reasons and partially because Togo is both hardier and softer. My SA convinced me that I should wait on contrast stitching for another SO and make this a classic black bag, so I did contrast only inside.
OP, I don't know if this is a first Kelly or a 30th, so can't assess how it'll fit into your wardrobe, but if you want a bag that will go from jeans and a t-shirt to an LBD, forgo the contrast stitch but keep the other specs. I think a black 28 sellier is great for evening. If you ALREADY have a small black K then by all means do the stitching. Have fun with the process as it is an amazing one, and try to get your preferences well set in your mind before going in to go through your choices.