what a lovely idea! I've always had a soft spot for those vintage suitcases, covered with stickers! and at twelve, it's just the age when you really start journaling with detail and sass!
If you're not 100% confident of your bookbinding skills, I'd say definitely buy a decent album or journal. I recently bought one for someone with that purpose in mind--a nice, spiral bound sketchbook. the spiral binding would allow her to fill it up with momentos without stressing a traditional book binding and also make it easy to lay flat to write notes. if the journal you choose already has a "manilla" looking paper cover, it'd be pretty easy to stain it with tea or something to look more like an old suitcase and scuff it up with sandpaper.
if you decided to go with a more conventional journal, you could buy or make little belts to look like vintage suitcase straps (or even buy a purse from the goodwill to deconstruct for that purpose). you also might be able to find a handle in the craft supplies, or make one out of rope or a strip of leather?
if you want to make your own "faux" leather cover, one thing I've tried recently that was really cool was to paint textured wallpaper to look like tooled leather, kind of like this: http://www.creativewallcovering.com/28cd-525205-wallpaper.html
I painted it with a dark brown first, then washed it with a lighter brown on the high points. it looked great! I found my paper at Meijers (a department store like a super k-mart or target). it's also great for other projects, even just covering ugly boxes you have lying around.
dover books has a book of vintage travel labels: http://store.doverpublications.com/0486421945.html
. they have it as stickers or as a cd-rom.
make sure she has a great pen! it doesn't have to be expensive, but something that draws and writes well.
Reader's Digest published a great book called "Sketching School" that would be very useful and inspiring for travel journaling. I got my copy used on amazon for less than $10.
I think the most important thing is that it remains functional and sturdy.
it's a wonderful idea! it sounds like she's a pretty cool girl, too.