Congratulations!
If your measurements are good I wouldn't worry about being small - only one of my pregnacies has given me any kind of 'normal' big bump, the rest just snuck up and hid under my ribs I think! I never looked more than a few months until they dropped down to be born!
I also have had a surpirse early arrival (mostly due to my own disorganisation it has to be said) and had nothing ready because I thought I'd have ages once I was on mat leave to sort it. You really don't need a lot for a baby, lots of the stuff we're encouraged to buy is a bit pointless.
Make 2 lists - one for you and one for your baby. The only thing I would say is even if you are sure you won't have any issues with BF and don't want to buy steriliser etc - get enough ready made formula (with a good date) and pre-sterilised bottles to get you through the longest time a shop will be unavailable in your area. If you don't need it it can be donated but if you do need it, you will really need it and unexpected hiccups can happen to us all.
You will need more initially, going into the birth. Don't worry about getting 'everything' soon after the birth you'll be wading through gifts!
I didn't even get a pushchair, just a sling.
My essentails were - carseat, nappies (forgot about wipes and ended up using cotton wool for ages!), clothes that could be layered - I didn't get one hot garment but lots of slip on/offable things and blankets it made life a lot easier for me and one of those bedside bassinet things that clip on to your bed (which I loved). I didn't get a bouncy chair because I'm a great big hippy and had baby in sling and came in bath with me but by the time I had to worry about where safe to put DC we'd had plenty of time to get things.
Don't forget you have Christmas coming - be shameless! We got a lot of little things and some bigger things were shared between people. You will probably get so many muslins you could make a bedspread.
It's a bit mind bending, particularly with such a surprise, but I can't tell you how much stress it saved me to keep it simple - does baby have a way to get home from hospital, somewhere to sleep, some clothes, nappies and a food source? The rest of your time can be devoted to you and your partner and getting used to things. Best of luck with work.