It is, of course, completely normal to have "slight asymmetry;" if anything, it's abnormal NOT to have that.
That applies to every part of the body for which there are two - including eyes, sides of the face, etc. You will most likely be slightly asymmetric after the surgery, too, and that - again - is entirely to be expected. For someone like you I would simply say that you'll be larger than you are now, and with a nice enhancement versus your current appearance.
It's understandable that you don't want to do anything about the discrepancy in nipple height (though that's hard to see on the photo you provided). As with the rest of the breast, I only suggest an "adjustment" when there is a fairly pronounced degree of asymmetry.
Of concern, though, for you is that implants have a certain amount of weight (450 grams is a pound) and you already have a bit of ptosis (sagging) - sorry! and nothing personal - so you expect that to be slightly more pronounced if you add what is essentially a one pound weight to each side. Not that you shouldn't do it - I think you'll look great - but that's just something else to be aware of.
And finally, on the size, that range seems like it will fit on you (only your own PS can make the determination in fact) and should be assessed at the time of surgery. I would urge you not to be "set" on an exact size. There is very little difference from one breast implant size to another. 25 cc is less than one ounce - the average size shot glass in the United States is 45 cc, or about an ounce and a half.
So I think you're on the right track. You can go over the sizes again with your PS, but it seems like a good plan, and I would not worry about the asymmetry, at least based on this photo.
I hope that this helps, and good luck,