Basic things like headcollar & leadrope, grooming kit, first aid kit, buckets, hay net, mucking out tools and hi viz for hacking could be bought before purchase. Then other things that should be fitted to the particular horse eg saddle, bit, bridle and rugs will need to wait until you have the horse.
will you be on DIY?
If so feed bin (usually the large plastic dust bin) can also be useful. I also have a large zipped bag for storing rugs (made by Shires) that's very useful. Large tubtrug for water bucket.
EVERYTHING! I loved shopping for my very first horse!
The way I did it was to get a Robinson's/Shires catalogue and just work my way through it making a list of everything I wanted. Then I went back through it a couple of times and crossed off everything I didn't need :/ Obviously your budget will have the final say, but either way, enjoy it!