I currently have three ball jointed dolls (BJDs).  I would love to get more but they can get pricey. You also have to factor in clothing, wigs, shoes, and other accessories.  When considering all of my purchases I made sure that my dollies were about the same size, so that they can share clothing.  Everything is interchangeable, and everyone can basically wear the same clothes.  The only difference in my dolls is in the foot size.

Miku #1
Gen X Naomi
Doll Head Blue Blood
Doll Body Jie Doll
Face Up included / Human (smooth) Skin

Saya #2
Chloe Minifee
Face up included/Normal skin (Not Altered)

Mitsuki #3
My Dolling HeeAH
Normal Skin (Korean resin normal)
Type No. 4 Face Up