I'd use a longline rather than a retractable lead. Does the rescue have an onsite shop? If they do, they should be able to fit her with a harness for you. You could probably purchase a lot of other stuff from there as well.
I would ask the rescue if she eats and/or drinks out of a raised food and/or water bowl because you may need to buy a stand for her. Some dogs are picky about what type of bowls they will use (another question for the rescue).
You'll need something to secure her in your car.
I would also ask the rescue which food toys she likes and understands how to use, and purchase them. These can help keep her entertained if you're not planning on walking her for the first few days, whilst she settles in.
If you're planning on feeding her something different to what she eats at the rescue, I would buy some of her current food and gradually change her over to the new food, to try and prevent her from getting an upset stomach.
I'd purchase grooming tools: an appropriate brush and shampoo, e.g. fox poo shampoo if she's prone to rolling in it 
A cover for your sofa and/or car seat if she's going to be allowed on them.
Towels for drying her off, if you don't want to share towels with her.
Poo bags, a head torch for night time toilet trips and an enzyme cleaner for any accidents.
I would give the rescue a blanket that smells like you and ask them to keep it in her kennel. She can bring it home with her when she's adopted and take comfort from the fact that it's familiar to her, and smells of both you and her.
Don't be afraid to ask the rescue lots of questions about her likes and dislikes. They'll appreciate it!