Personally I usually go with The Lady for stealth builds. However, with this build you've got nothing based on personality. I think speechcraft is an excellent RP choice no matter if you are agent, assassin or thief. YOU VILL TELL ME WHAT I WANT TO KNOW!
One other thing, armorer is nearly never a poor choice, but consider the light weight of short blades. You can carry a couple of them with no problem.
Either way, since about 50% of Morrowind's population is Dark Elf, even without speechcraft you'd be well liked around the land for the most part.
So, I say the lady. Optionally I'd swap armorer for speechcraft. At that point, if there is another minor that you'd rather was a major other than speechcraft, then swap speechraft in to a minor with whatever skill you choose.
The only birthsign that one should never take is the one that gives Turn Undead on both target and touch (the ritual?). These powers are worthless junk in my opinion.