Some of these items are really not necessities, but more personal preference. I personally do not use bell boots, polos, or any form of boots on my horses. I do hunters though.
Not every horse NEEDS supplements. I would recommend getting your vets opinion on what supplements your horse should be on, if any. As for the wormer, again you need to ask your vet on this. Some vets will recommend a different wormer depending on your location/time of year/etc. The vaccinations are for your vet to give, they are not something you will have unless you do your vaccinations yourself (I do all my vaccinations myself, but I would not recommend this for a first time owner at all, you really need to know what you are doing - it's not something to play around with and/or just jump into).
I would recommend buying more than one saddle pad, and also not buying saddle/bridle/bit until after you've bought your horse and know what size bridle/bit, as well as what type of bit is needed. I would also highly recommend a saddle fitter to come out and try saddles on the horse to find out what fits and will work best for both you and your horse.
Unless this is something to do with your area, I don't see why you would need a bot fly knife? Can't say I've ever owned or needed one.
Another point would be does this horse need blankets? If so then you'll need those as well. Some horses won't require blanketing, some do. Both my mares have a full set for year round (Heavy weight, mid weight, light weight, stable rugs, no-fill rain sheets, fly sheets, coolers ..) I wouldn't buy any of these until you have the horse and know what size the horse takes/what brands fit and which don't/which blankets you may chose not to have, etc etc.
My first aid kit is a mile long (mostly things I have needed "one time" and keep on hand "just in case" like tweezers, syringes, polysporin, diapers, epsom salts ...the list goes on and on and on.) But, one thing I do want to recommend though would be keeping Vetericyn on hand. It works like magic and I will always, always have it on hand.