Medieval Lady Death would be easier to cast, you could just take any garden variety actress, bleach her hair and give her contacts. Normal acting ability would suffice.
Now the classic Lady D is much harder, not only do you need to find a 6'4 amazon with boobs the size of mount everest to prance around dressed in the amount of cloth itd take to sew a single glove, but you actually need someone who can be convincing at the same time.