Hey B.!
I am a working mom of 4. My oldest was 4 and my twins were 1 1/2 when I had my daughter. My main advice to you is YOU CAN DO IT! I got very distraught when I was pregnant with my daughter thinking I was going to have to be put in a nice soft room in a pretty white jacket when I found out I was having my daughter! lol ;> It was tough at first but the best things are the toughest things to get to. It will be so worth it, I can not imagine our family without her as each child adds there own part to the family and balances it out.
Something that helped make it easier was taking time to introduce my daughter to each of her siblings as THEIR new sister- invoke the protectiveness and helpfulness inate in children and be sure to give them LOTS of ATTENTION when they do nice things for their new sister or to help you. Get a pamper/wipees for the baby, put the bottle in the sink, get the bottle for you. Include them and give them lots of hugs. Although you have to be careful with the new baby, try to not be afraid to let them explore their new sister. Re-direct if necessary- if they are touching the babies head/eyes/etc redirect by saying look how nice you are being with your sister- she likes when you play with her toes or rub her legs. That way they don't feel rejected but are not poking the babies eyes out either! Baby slings saved me, because that way I could be holding the baby and still have my arms available for hugs and I knew they weren't experimenting with her while I was making dinner.
I had a GRACO double stroller- side by side that I loved- huge basket underneath and very light and easy to fold. I carried my daughter in a sling.
We have a family bed so I didn't have to transition anyone, but I did use a Arm's Reach co-sleeper for the baby which attaches to the side of the bed so the baby is right next to you but no one will roll over her. By the time your littlest is ready for a crib your other daughter should be ready for a bed. Also good because the baby won't wake up the others if she is next to you and you will hear her before she gets going full force. You want to avoid having to deal with grumpy sisters getting up at night too! You can also put a soft sounds cd in their room so if the baby does cry they won't hear her.
Lately I have been getting up earlier than the kids to have time to take a shower in peace and maybe a cup of tea if I can squeeze it in. That helps alot to have just some time for you.
Just remember it gets easier as they get older so the roughest time is the first few months while you get used to it and used to lack of sleep. Just don't think about it or compare it to before you had kids! Stay in the moment and you will be just fine!
Have fun! May you and your family be blessed with lots of patience and love!
A. G