Frokky, the 18-55mm kit lens that comes with the 400D is pretty good. There’s probably no need to replicate that zoom length with one of the 3rd party lenses, unless you need one of the f2.8 versions (and unless you’re making a living shooting weddings, you probably won’t need f2.8).
Both the Sigma 10-20 and Tokina 12-24 get good reviews, but I’ve no personal experience with either so can’t really recommend one over the other.
I have a Canon 70-200 L lens and it’s an amazing lens, but with the cheapest of the 4 versions being about $22K that might be more than you want to spend. Although, any decent quality Sigma, Tamron etc will probably cost between $15-20K, so you might want to save a little extra and get one of the L’s. I’d actually recommend doing that - save an extra $5-10K and go with the 70-200 f4L.
Flash, the 430ex should be good enough. I have a 420ex, which was the model before the 430, and it does the job for me, but then I shoot with available light 90% of the time. You may need to check that you can set the flash in the camera - the film camera equivalent of the 400D doesn’t allow flash settings to be made in the camera. If the 400D/430ex doesn’t, you’ll need to either buy a 30D or a 580ex.
Also consider the Sigma DG500 flash. It has all the same features as a 580ex, but costs the same as a 430ex.
Bags - I have a couple. My main bag is a Lowepro Computrekker AW, in which I carry my 20D, 4 lenses (50mm, 17-40mm f4L, 70-200mm L, 100mm f2.8 macro), flash (420ex), 2 portable hard drives, various cables etc. It also has a padded space for a laptop, and an inbuilt tripod carrying system. I got it on sale for about $3500, but they normally cost around $7000.
Another bag I put together myself came from an outdoors/camping store. A small waist pack, which fits the 50mm and 100mm macro, plus batteries, CF cards, maps etc. Attached to that is a padded bottle bag which is the perfect size for the 70-200mm, and the camera with 17-40mm goes over my shoulder (if I’m walking around with my camera, it’s always out, turned on and with the lens cap removed so that I’m ready for anything). This setup cost me about $700, and is great for summer as it saves a sweaty back.
I also have a smaller, camera store branded backpack from a store in Bangkok. Good for short trips.
And yeah, I think that website is the one I was thinking of.