My very first thought is to move your office down to the guest bedroom and add a futon or day bed for guests. Putting a little girl down in the dark basement doesn't seem like it would be so great for her.
It is so hard on the first child to take in the second or even the second to take on the third that sharing a room I found with all three of mine does not work so well either. Not only do they have to share you, now they have to share their room, their stuff, etc. Not to mention the fact that the baby WILL wake up the toddler even if you keep the baby in your room for the first couple of months.
All that being said, here is my suggestion. Keep the toddler where she is, move the office to the basement, put the baby in the current office. I know how hard it is to move an office, especially since it is probably your only spot for just you and your husband and I am sure it is packed to the brim. Here is How I do it every time I need to move stuff around. You have a lot of time, so start with the guest room, move things out a bit at a time. Each day, dedicate 15 minutes to a half hour to moving the room out. It may take you a month but take your time, the baby isn't here and even when he/she is, you have a couple of months before the room has to be done anyways. Once you have the guest room all cleared, I like to do something special to it, like paint it, ad an area rug, something to make it mine. Then I proceed to take the current space (your office) and move it in the same way you moved the guest room out. Spend 15 minutes to a half hour each day and move a bit at a time.
One big rule to this is, do not take out more than you can put away. This will narrow down the mess so you don't feel overwhelmed. Basically by the time you have everything moved you will look back and think, wow that wasn't so bad.
Just so you know. I do have two of my children sharing a room at this point, however they are older, more independent, etc. I was never able to have a baby share with another child no matter what age the older child was.
Good luck and congrats on the new baby!