Yes, you can. It has a Hot Shoe Mount.
Yep!
Canon replaced the 20D with the 30D that had more mexapixels and newer features. In fact, the 30D was replaced by the 40D, which was then replaced by the current (Jan. 2009) 50D camera body.
Yes, search for "Canon 30D Lens."
You can buy a Canon camera at most local department stores such as Walmart. You can also buy them at local photography stores, or even online retailers such as Amazon or Ebay.
Absolutely. The 430EX was designed to be a cheaper speedlight option for Canon SLR users. It was especially designed to work with the 5D which doesn't have a built in flash. If you buy a 430EX it'll interact with your 30D, 350D or similar EOS SLR to help with white balance and focusing as well as giving you more professional looking flash photos. The flashgun will read the zoom length of your lens, how far it's been zoomed and other info from the camera and automatically adjust it's power and throw to provide the right flash angle and strength for each photo.
Here's the camera (body only) for $650 http://astore.amazon.com/techr02-20/detail/B000OG1J14
All Canon Rebels can use either Canon EF (or compatible) or Canon EF-S lenses. EF lenses can be used on any Canon EOS camera. EF-S lenses, introduced in 2003, are made specifically for Canon EOS cameras with APS-C size (22.2 x 14.8mm) sensor. They can be used only on Canon Rebel cameras and on the Canon 20D, 30D, 40D, and 50D cameras (as of 2008). No other manufactuer currently makes camera bodies that take EF lenses. (Several years ago, Kodak had a full-frame digital camera that took EF lenses. It was a camera that was OK in the studio, but inferior for general photographic use. It was discontinued shortly after introduction.)
Yes you can if your taking a trip to Britain you can put it in your luggage.
Yes, checkout canons website, or any photo site to buy a multitude of external flashes.
Depends, I Have the 30d and i havent found one. But even if my camera got wet or even ran over by a car, its still covered. insurance baby :) its worth the investment if you have such an expensive and awsome camera like canons.
The short answer is "Yes." The long answer is: All Canon EOS cameras use Canon EF (electro-focus) lenses. They can also use independently made lenses with EF type camera mounts. All Canon DSLR (digital single lens reflex) cameras can use all Canon EF (or independent manufacturer equivalent) lenses made for film or digital cameras. Most Canon DSLR's have an APS-C sensor that measures 22.2 x 14.8mm. (This is smaller than the 36 x 24mm image size on 35mm film.) In 2003, Canon introduced EF-S lenses that were optimized for the APS-C sensor size. The EF-S lenses will only fit on Canon 20D, 30D, 40D, and 50D cameras (as of 2008) and all Canon Digital Rebel cameras.
Yes, the BG-E2 battery grip will work on the Canon 40D, 20D, and 30D. It has been replaced by the BE-E2N, which supposedly has better sealing around the battery compartment.