Hi PorcupineRim - congrats on your new Westy! Your best resource is probably the Yahoo group here:
The photo section (once you've joined the group) will have good ideas for you for rear storage / bike racks / tire carriers.
Regarding the shower, you should remove the old caulk as much as possible. New caulk doesn't like to stick to the old stuff. An x-acto knife is useful, but I've found the best tool to be a metal scraper, like this:
http://www.google.com/products/catalog? ... EPMCMAc4Cg
Go to your local hardware store and find one that's sharp. Use the knife to cut away the bulk of the caulk, and then the scraper to get the remainder cleanly off of the surfaces. I also like a caulk application tool - basically a little shaper to make your bead look nice and clean. Also available at your local hardware store