Providing you will find their http://www.uesp.net/wiki/Morrowind:Restore_Magicka#Alchemy_Ingredients, which isn't easy at the start of the game. I'd go with The Lady for the high personality and regenerating magicka whatsoever, because it's toughest to exist in the world at start. Yes, The Atronach is superior later on, but later on it's easy anyway, so...
I don't say anything on the role playing aspect though, since this guy is half-and-half a mage and a stealth character anyway. Besides, how can you roleplay with your birthsign? Do the birthsigns affect so much to the choice of profession that the TES characters are basically at their mercy? I'd like to think they choose what they like, without caring that much about their birth powers. Of course these powers might influence them towards certain professions, but I think the choice is ultimately the character's, not sign's. Therefore, it's all the same for the narrator (player) to choose any sign - preferably the best fitting - and it's not that much role playing, IMO. It's more a 'coincidence' in the game level. I'm a Cancer in real life and... so what? I'd take pretty much the same attitude if I was a Morrowinder.