Hi H.. These days, Pediatricians support keeping the kids in cribs as long as possible and allowing them to drive the transition schedule -- so it's difficult to tell when your daughter will be ready. But I can see how frustrating it would be to buy a new crib and then have your child ready for the transition only months after!! Could you possibly purchase a used crib for your daughter until she shows you she is ready for the transition?
In terms of transitions in general, I strongly advise making any significant transitions for your daughter as far out from the birth of baby as possible (either before and after). My kids are 15 months apart and we tried to keep everything several months out from the birth, so that my daughter would not associate (or blame) the changes with her new brother. (For example, we took our daughter off bottles at 12 months, removed her from her high chair into a booster chair at 13 months, etc.)
The last thing we wanted was her feeling even more stress from changing the common things in her life along side of having the new baby in my arms. Our rule was: anything the new baby will be using became off-limits for our daughter months before the baby came home. We stored the baby swing, the high chair and the exersaucer out of sight and we removed all the early-baby toys. She "forgot" that they were once hers and had no possession problems when her brother started using them!
Best of luck with your growing household!