I'm so sorry to hear this. It sounds like a very difficult decision to make. We had just a taste of this (baby has less than a 1% chance of a rare disorder) and it really knocked me for six and I struggled to connect for a long time so I can't imagine how it feels to know that your baby will have to go through surgery etc and might not have a chance at a longer life.
I think if it was me I'd buy the absolute essentials - car seat (just the seat, no base) or possibly reserve one from somewhere like Mothercare so you/DH can pick it up if/when you come home from hospital, place to sleep you have, a few clothes, nappies, hospital bag stuff. I don't know if I'd get a pram, just because they are so bulky and expensive if you don't know. Maybe a sling because they are cheaper, but also because it will be nice to keep him close to you and could even be beneficial for him. (You may want to look up the benefits of kangaroo care). I'd get a stretchy wrap as I think those are the easiest and best with newborns and especially for kangaroo care as they cover so much you can go without clothes underneath.
Then I would definitely spend some time getting some sentimental things, nice clothes, toys, books, a special blanket. Not spending a fortune and not going past things you'd use when he's really little and new anyway. Because if the worst happens, it will be nice for him to have things which are just his, and which you can choose whether to send on with him or whether to keep as a memory box/to use with any future children you might have to keep their brother in your thoughts. These are the things which are fun to shop for anyway so it might help to buy them, and it sounds quite likely that you will definitely have the early days at least so I would focus on making those days special and then take every milestone as it comes.
Everything else you can just pick up as and when you find you need it. TBH I think that a lot of things you are told you "need" for babies are total hokum anyway. Wait and see what you want - when you come home if he doesn't settle without a swaddle, or you run out of clean clothes or you find you need this or that cream you can get most things next day delivered online or go to a big 24 hour supermarket. If you find you've nowhere to put him down you can order/go out and buy a play mat or a bouncer. If breastfeeding is tough and you want to give a bottle of formula, same - etc.