It's going to take some work to get wasps arms into the Buffy shoulders, the pegs are very different.
I don't think reusing anything but the heads from the original xmen figures is worth it, Jean has a excellent resemblance, however storm and rogue didn't fair as well.
If it were me, I'd use GOTG gamora for storm and jean, and movie sue storm for kitty and rogue. The Gamora head would probably work pretty well for storm, or maybe even the scarlet black widow head.
I'd have said movie black widow would be a good base, but she's too curvy. There is a movie Maria hill figure coming out that would be pretty awesome of a base for Jean and storm.
Well good luck, hope that helped