I would not buy a new bike. Rather, I'd use an old bike I patched up or something given or lent by a family member. Something fairly sturdy with enough gears, not too worn out so it will make it to the end. Probably a mountainbike, preferably one with a stiff fork.
Riding an old bike is cheaper and will tell you something about what you want, what your style is. This will help you pick a top-quality bike in the future. It would be a shame if you bought a new 26" frame but find out you'd rather have a 700c or even 29".
With regards to all your equipment, I'd say your main concern is durability. Don't buy a brooks saddle. Really, at this point it is not worth the money. When I bought a saddle, I first measured the distance between my pelvic bones by sitting on a piece of cardboard. Then I measured the distance between the dimples I'd made. This seemed to fit a cheap saddle sold at Halfords. The saddle had a plastic cover and a thin layer of gel/foam. Never had a problem with it, not even during a three-month tour. I do wear chamois though, but at the big discount sports stores, this also doesn't have to be expensive. I like the decathlon chamois.