Don't worry! I bet things will go smoother than you think. You sound like me, I want to be prepared and I always prepare myself for the worst so that I'll be surprised when things go well! Your daughter also sounds a L. like mine, independent, knows what she wants. My daughter was also 20 months when my son was born. I didn't even begin to go there with the potty training thing. I had a potty seat and if she wanted to go on it, I encouraged her, but I basically didn't say too much about it. I knew that once the baby was born, I wouldn't be able to give my daughter as much attention on that end as she deserved and luckily, she wasn't truly ready anyway. She is now 3 and for the past month has been completely potty trained (diaper at night though).
I would just talk with my daughter about the baby in my belly and let her help us get the nursery together by letting her put clothes in the drawers and put diapers in bins, etc. We asked her opinion on names, just basically made her feel like she had a say in our her new brothers future! Once he was born, I let her help as much as possible and I always asked her if she wanted to help instead of expecting her to help. While I was nursing my son, I would tell her to go and get some books and I would read to her while I nursed.
My biggest challenge was getting her to stay quiet while I was trying to get my son down for naps and bedtime. She just wanted to be in the room with me but didn't know how to play quietly. Instead of getting irritated with her, I just gave up and let her play. My son either wasn't bothered by her or just got used to her noise (he was going to have to get used to the noise anyway!)
The only time I got worried about my daughter adjusting was when she came into the hospital to see me. I made sure that I put my son into the bassinet so that I could immediately hug her and show her her new brother. Well, when my husband brought her into the room, she stopped at the door, stared at me for a second, burst into tears and ran away!! I guess she wasn't expecting to see me or something, I don't know. My husband ran after her and walked around the hospital for about 20 minutes until she was ready to come in. When she did come back I just let her get acclimated first and let her come to me. I spent some time with her and then asked her if she wanted to see her brother and things were fine from there. She did incredibly well at home with daddy, I'm a SAHM so I thought there might be a problem there since she was used to being with me so much. I am lucky in that my daughter is one of the most easy going kids I have ever known. She adjusts so well to everything and I admire that quality in her so much. I hope she keeps that forever. The bottom line it, just let your daughter be her own person and don't push her. Ask for her help and let her come to you.
Oh, one more big thing!! Work very hard to get the kids to nap at the same time!!!! I know for the first few months that is impossible, but once your baby is older and on a more predictible schedule you will cherish that time so much. My son is now 19 months and my daughter is 3 and every day, I have 2-3 hours ALL TO MYSELF!!! WOOO-HOOO!!!!!
Congratulations to you and don't stress, you have enough to worry about! Sorry this is so long!!!! : )