Prickle I used to have a harness like this: Harness
for my big dog, until I changed cars. He now goes in the boot happily. The rescue centre can advise you on what is best for the dog you choose.
As for grooming stuff etc - I have some good brushes and you'll need to flea treat the dog - but all of that can come with time, don't need it straight away. Same goes for nail clippers - they're great if your dog is ok with you cutting its nails. If the dog is nervous, it's best not to try for a bit. Also, if you walk the dog on tarmac, you may never need to cut its nails.
I'd recommend going to a charity shop and stocking up on old towels, and old blankets. Good to have a few handy for bedding, drying after dog walks etc.
Re Crates - they are great if your dog is nervous, or naughty when you're out. It gives them a safe home that they can retreat to if they want some space. If you have room for one, I'd recommend getting one.
Re toys - I agree, don't buy expensive ones to begin with. My dog is happy with a plastic bottle, or a tennis ball stuffed into an old sock. It depends whether your dog is a chewer or not, or a chaser, a tugger... they all like different things. See what the dog is like first, maybe buy some really cheap bits like frisbee etc and take it from there.
As for online shopping - I know I shouldn't but I tend to go to Amazon for pretty much everything. But there are lots of independent online retailers out there too.
Well done you for getting an older rescue dog - all your dog really needs is your time, attention and love.
Enjoy.. and be prepared for your life to change forever! (in a good way!)