Something to note...you'll need a protective vest for cross country. You can get away with doing your dressage test in a non-dressage saaddle at the schooling/low level shows but they do require a vest for the cross country portion of your show.
Eventing is a BLAST! I did it for 3 years, it's what I learned to jump with (and my horse too!), we just recently, ironically enough, switched to Hunters because dressage was starting to get too stressful for my built-on-the-forehand long and low mover of a horse!
So Hunters suits her much better! But I love cross country, so we still go on Hunter Paces to get our fix!
Feel free to PM me if you have any more specific questions about Eventing!