A.,
I am the mother of a boy who will be 5 in June and girl who will be 3 in April. I belonged to a mom's group with my son and when our boys were turning 1 year old, we had an Early Childhood Development Specialist come talk to our group about normal 1 year old behaviors and any questions we might have. Someone raised the issue of when to move our little ones into "big boy" beds. She said it is best to keep them in cribs until the age of 3 if at all possible and at least until 2.5 years old because they simply are not ready mentally or emotionally to be in a bigger bed without the rails any earlier. Kids who have been sleeping well throughout the night for a long time will start to have sleeping issues if you move them out of a crib at an early age. I kept my son in a crib until about a month or so after he turned 3. We had a crib on which the one side would come off and we took that side off right around his 3rd birthday.
With my daughter, we had every intent of doing the same with her. She is in fact still in a crib, but it hasn't had the side on it since last July when we discovered that she was climbing out of the crib. The only reason we took the side off was because we had no way to keep her from climbing out of the crib and it was a safety issue - she was doing it even with wearing a sleep sack! At that point, she was not quite 27 months old. We had some difficulty in the beginning with her getting out of bed, primarily at nap time, so we just had to work with her to teach her she still had to stay in bed even though there was no longer a side to keep her in there. She will be getting a regular twin bed soon as she is nearly 3. I wish we had been able to keep the side on the crib until the last couple of months as we did with my son. Everyone I know who has tried to move their kid(s) out of a crib before the child was at least 2.5-3 years old has had problems. A couple have even moved their kid back into the crib it was so bad!
My advice is to get another crib and keep your daughter in her's for another year. The price of the second crib will be worth it. You're already going to be up in the middle of the night with a newborn and exhausted. You don't need to be trying to retrain your daughter how to sleep in her own bed without you being there to make sure she stays in bed, to alleviate any fears, etc. When it does come time to transition her, there are a couple of things you can do. If you have this kind of crib, take off the side to get her used to being in a bed without railings. Talk to her about it before you decide to take the side off the crib and before you get the regular bed. Explain that she will no longer have railings to keep her in bed, but that it's just as safe and actually better for getting up in the middle of the night to go potty. Take her to shop for the bed. Let her tell you what height works best for her. Also know that you do not have to get a box spring right away. You can get one of these boards to put under the mattress and provide the support a box spring would provide without raising the mattress so far up. This makes it less scary and easier for the child to get in and out of the bed. Also, if she does fall out of bed (likely at least a few times the first couple of weeks), it isn't as far to fall. Try to set the bed up in her room for a few days to a week before you actually move her into the bed. During that time, spend time with her just hanging out on the bed and getting used to it.
Whatever you do, good luck! And congratulations on the pending arrival!