Skip it and go straight to the manga. The first anime (Xebec) sticks with the original story until about halfway in where they ran out of source material and did an anime only ending. The second anime (Shaft) is completely original and has absolutely nothing to do with the manga. The OVA's are just random chapters of the manga you wouldn't understand unless you've actually read the manga itself and the movie is also original.
That ended up being way to long...
Basically...
Well, considering she's still in school, she's definitely not a "legal" loli, though in this case, I'm pretty sure it's more about her body size than anything.
Was it just me, or did Stacy get shafted in the CG department?
EDIT: Never mind, just looked and she had more than I remembered.
Still a fun character though. Hope her route gets translated one day.
Not sure about the cyborg girl, it's been a long time since I watched the show, but the sisters are in the game.
This is also why I don't watch dubs very often, I usually can't stand the voices.