Not to be a pervert. Ok, I am but don't forgot to show pictures. I wouldn't believe you that you made this prop if you didn't show a half naked lady during the castings.
Body casting is not the way to go with this project... WAY to expensive for a one-off project... chicken wire, fiberglass and perhaps some modified maniquin pieces... first off the position of the body is way to complicated for someone who has never done body casting, second it is not cost realistic to do it this way unless it is for a production run...
Let me see if I can break this down.
1. You are hoping to recreate the mourning angel monument from the picture you posted. You say you want to cast small female parts. What scale are you considering? Are you hoping for a small woman? Small child?
2. You are wondering what medium to use and how to finish the piece so that it will look like sculpted marble.
If I were going to build this, I would use a combination of styrofoam (for the sub-structure and block portions) and plaster for the casted body parts. To get a smaller scale, I would actually use either my 5 year old or 12 year old daughter's arms and feet. I would just make a quick negative and then pour in plaster for the new positive. For the statuary clothing, I would use either MonsterMud or GP Resin.
To finish the piece, I would give the entire piece a really good basecoat of exterior white primer/sealer. Then I would go to a home improvement store and get an appropraite faux marble kit. There are a lot of different onces to choose from depending on what kind of marble look you want. You might also check a theatre site or two and see what they say about faux marble painting.
I would then finish the whole piece with a coat or two of gloss or near-gloss clear finish. This will enhance the look of the marble and give the piece a smoother looking finish.