I am far from experienced, but here's my two cents. I took my pony out to our first paper chase (first time schooling solid jumps) with no boots at all. He's a large pony and finding boots his size isn't the easiest thing. We have since bought boots to use, but my instructor actually advised open fronts even for XC since he's still fairly new to jumping and rather on the lazy side, she said he needs to feel it if he hits something, and we aren't out doing 8' tables. We also had to modify those boots after getting them because they still didn't fit just right. If you're going to buy something for XC, make sure it doesn't absorb too much water and is well ventilated, but if you're only going to school a course or two a year, all small, it may not really be necessary.
If you're doing just a schooling show, people should be pretty willing to help out. Ask the organizer when you get there if you know you have questions, or just inform them this is your first time, what do I need to know? Ask other competitors if they don't look too rushed. At most of our local events, they have open schooling on Saturday and a schooling show on Sunday. If you school the course on Saturday you aren't eligible to place on Sunday, but it's all for fun anyways, so I say school it first to set your horse up for a good positive experience. I've been able to find a pony club instructor who gladly took me under her wing and I went out schooling with her group so Pony could watch another horse do something first, by the end, he was happily cantering out front taking jumps first.
We don't always have numbers, or readily visible numbers, so I introduce myself to the volunteers that are outside of the warm ups or calling up people when we get there. It's much easier for them to flag me down when they know, oh, rider on the white pony with the blue pad, rather than trying to see that little number on his bridle.
Other than the XC vest, I feel like most of the things you need to pack wouldn't change from a dressage show.