i work, I go out to meet people, I do activities. Which all require clothing.
So do most women, and they manage not to need an overflow room for their clothing.
You need to have your DP go through his clothes and gear. Either your bedroom is the size of a matchbox or you both have far too many clothes.
When DSD comes she brings a bag and I clear out my dressing table for the weekend and just use a makeup bag. 3/4 weekends I have the luxury if using my little dressing table , because again I like to take pride in my appearance. It was my little bit of luxury but was always going to go.
I get my face on in the bathroom, using my makeup bag to hold everything necessary. When I was a sahm I had the same makeup bag. With a baby, you are going to feel lucky to get a shower in to start the day.
So DSD's room always had a duel purpose. Whether it was a guest room or DSD was staying or simply my dressing room. I do feel awful , but my baby is coming with clothes and cribs and matts , I need the space of the spare room.
Babies have one crib, one mattress, and a limited number of clothes they fit into at any given time. You do not need to have a year's supply of clothing, bedding, footwear for a baby. You'll need space for nappies under the crib.
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I would buy a single bed with underbed storage for DSD to put her things into. I would put the crib in the same room. You should plan on having the baby in your bedroom until s/he is sleeping through. This might take months.
Please don't buy a foldaway camper bed. That underlines the message to DSD that her father's house isn't her home.
An actual single bed in the current dressing room would be very useful for you when DSD isn't there, for all sorts of baby-related matters.
Please try to involve DSD in the decoration decisions of the room she will use and where her baby half-sister will be sleeping. To insist decorating decisions will be yours and only yours sounds really hostile and non-inclusive.
This is an ideal opportunity to bind the family together and facilitate a positive relationship between the two children, which you seem to be determined to prevent for some reason.
Use the box room for your office and any extra storage needed.
You need to get rid of the double bed, obv.