I think in the past, most Pullip customizers bought the 27cm obitsu body to upgrade their girls, but since the 25cm bodies came out folks tend to prefer them since they're about the same size as the default pullip body. So it depends on whether you want your girl to be the same height or taller.
The 23cm and 21cm bodies are more Dal/Byul sized.
And of course if you want to swap her gender, go with the Obitsu Slim Male body!
Putting a pullip head on an obitsu body does take some customization, though. RequiemArt has a tutorial for hybridizing the older head types:
http://www.requiemart.com/pullip/swap3.html I'm not sure if type 4 requires as much work or more.
Hope some of that helps.