I have done eventing now for 5 years, but I started as a hunter. Let me tell you- one event and you're hooked- no more hunters! :) Here are some of the things I really had to work on when I made the switch:
-Bend your elbows and have them relaxed. Bend them when you sit and straighten when you post up.
- Relax your back. There should be no arch, more like a soft curve to a relaxed flat back with following hips. Think about sitting between your seatbones and your tail bone with shoulders back and just barely feeling your seatbones move with the horse.
- Your heels don't have to be (and shouldn't be) that far down. In fact, you're more balanced with a relaxed heel than a braced heel stuck all the way down.
- Make sure your horse can bend, get on the bit (on the verical is best) and stay off his forehand (big problem with so many hunters)
- Don't perch yourself like a hunter. The best place to be is right smack in the center of your horse. If you feel him start to back off, you can easily roll your seat and start driving.
- Most people are in a 2 point for the ride between the jump, but about 3-4 strides away, sit and get ready to push if you need to.
- It's all about riding your horse. It's not staying out of his way like in hunters. In eventing, the motto (around here at least) is "doesn't have to be pretty, just kick on" which pretty much means- as long as you get over it, clear your head and start riding to the next fence
- Most people never sit down on cross country. Make sure you can stay balanced in a galloping position before, over and after jumps.
- Make sure you can stay balanced doing down drops or up banks
- Get your horse out. Let him see water, ditches, banks, drops, solid fences with lots of color.
-Have fun. This is what makes eventing different from other sports. Make it count.
As far as conditioning: spend most of your time on dressage and you will be fine. BN is not demanding at all if you are prepared. Even when you think your dressage is good enough, make it better. You will be surprised how many people compete several levels lower than they train just so that they can win.
I use front and back Woof brushing boots and I wear a Tipperary vest. Make sure you remember to buy the number holder. It's a plastic sleave that you wear like a vest and your number slides in there. Remember to get a medical armband (complete with your bloodtype) and a coat for dressage. Depending on the order of the show, show up for stadium ready for cross country. Make sure your helmet is eventing certified and you use a legal bit for dressage and stadium. I ride all three phases in a Dutton XC Saddle, so you will be fine. Make sure you know the rules and how to jump the jumps in warm-up(red on right). You will see some tough riding, so take note of that to use in the future :) Also- bring a crop- most of the jumps can make the horses back off pretty good.
I don't know about where you are, but where I am, eventers don't think very highly of hunters at all. I wouldn't mention that this is just something on your bucket list unless you want to sit and debate all day :) Good Luck! Let us know how it goes.