A leash, buy one that is strong well made and you like the way it feels in hand (or find used).
Long-line--you can find them cheaper at farm stores (for pony's/lunge line) or just DIY rope and hardware.
I make lots of things with rope and hardware. Hardware clips include strength rating. Don't buy carabiners without a strength rating.
Keep checking kajiji and other postings for used crates. Crates are useful, and expensive but if you keep checking you may get a good one super cheap when someone's decluttering their home. I have 2 excellent condition 'big dog' crates I need to get rid of, for instance.
I would not buy too much ahead of time. It's fairly easy to make a trip to the petstore. An escape proof collar if you're getting a rescue (aka 'flight risk)--martingale or limited slip. Scared doggies in strange new places can back out of regular collars quick.
Harnesses, I would NOT buy ahead of time. Get the dog, then the harness. If it doesn't fit, you want to be able to return it, right? Unless you find something used super cheap. I have a bunch of old big dog harnessess... yeah, if you were my neighbour, I'd be saying come on over and take your pick...
so to save money, if you know anyone with a dog, they may have some leftovers/extras to hand off...
Dogs can sleep on blankets, old pillows (or new ones if you don't mind), or your couch if you don't mind, or a carpet, again, no hurry on that.
Sweaters, warm gear, don't buy. Cut up used people clothes do a fine job, then you can go to the store and try on and fit a nice one with your new dog if you actually need one.
What you need for the first day/s. A leash and collar or some other means to secure your new dog and get him safely out for a pee.
Good food (and you can buy that last minute, it's not like petstores run out of everything all at once).
POOP BAGS--all caps because it's an easilly overlooked essential for even the first hour (you can even start collecting those from corner store purchases).
Bowls, no hurry. You do have dishes, right? Well okay, maybe you want to buy a bowl. Some dogs like to pick up and play with bowls so extra heavy ceramics from the petstore may be worth your dollars, or just wait and see.
I'm big on wait and see.
And I'm super cheap.
And then I go spend my money on fancy collars or agility classes, oopsie.
Oh, treats... my trick. If you feed kibble (controversial these days, but I do), get a zip lock baggie, fill, add yummy liquid, leftover gravy, milk, parmesan cheese, or just water & parmesan cheese (great for camping trips) let soak, and voila, yummy soft treat food.
Tug toys, old fleece pants, cut legs into long strips, braid, tie off at the end, done.
Enjoy the anticipation...