My advice is "Listen to what Bob recommends!" He will guide you through what size shafts are needed, what frame, what wheels, all that stuff depending on your individual equine and your body size.
I would order a whip holder, singletree, seat slider and grippy stirrups (many of those may be standard now, I'm not sure) and depending on the size of your horse you might ask him about the seat angle. I like mine angled back a bit and he arranged for that when he built my frame.
The pneumatic tires give a far softer ride and he makes them fairly puncture-resistant (I've never had one even lose any air and I've driven it across brambles for four or five years now) but you'll need the solid rubber ones if you're going to use it on marathon above Training Level.
Some people like to buy the "saddle bags" he sells, those came along after my time so I just wear a fanny pack to keep things where I can reach them without getting out of the vehicle. I used to keep my spares kit in a bag that swung beneath the seat before he came up with the back-of-the-chair model but it was hard to get to if your horse was being fractious and I found I really didn't need one. (There's nothing on the 'Bike a wrench could fix anyway!) For day trips I just keep a halter on under the bridle, tie the leadline up to the saddle or breastcollar, stuff a hoofpick, my camera, a small water bottle and my keys in the fanny pack and off we go!
I think you'll really enjoy having a Hyperbike. Let Bob know where you heard about it!
Leia