A lot of the things on your list can't really be purchased until you buy the horse.
Rugs are like jeans - it's not one size fits all. One 16hh horse may fit into a 6' rug, while the next will be in a 6'6 and the next a 5'9. Wait until you buy the horse to buy a rug.
Same goes for a saddle. Not all saddles will fit all horses. Some saddles will NOT fit a certain horse no matter how much you spend of a saddle fitter, because the tree is moulded to a certain shape that cannot be altered. Is your friend able to remove, repack and shape stuffing in a saddle to fit a horse? Or is she just good at picking if a saddle is not sitting correctly?
Don't buy a saddle until you buy a horse.
Bridles, well, up to you. But same deal as with rugs, a 16hh horse may fit a full size bridle, the next 16hh may fit a cob size and the next may fit a warmblood size.
Same goes for bits, bits for horses are like shoes for people. Some horses hate some bits while the next may love that bit. Sizes range hugely as well, a too small or too big bit will have a dramatic impact on how a horse travels.
What you can buy in advance, is the generic gear such as brushes etc.
You can put together a first aid kid, get a list of farriers, vets and dentists in your area, put a grooming kit together, make sure you have found a suitable agistment, or if you have your own property, go through your paddocks thoroughly to ensure there is no stray wire or any other matriels/holes etc. that could cause injury to your new horse.