I see this question a lot. What are you wanting to do that you cannot now? I assume you have the 18-55mm kit lens. If you need wider angle, then you should be looking at a focal length shorter than 18mm. None of those in your list fit that description. You need something that at the wide end is in the 10-12mm range to get you any significant width. All the lenses in that range are going to set you back at least $600 or so. Canon's 10-22 EF-S version is excellent. Sigma has a 12-24 and will soon have a 10-22. Tamron has a new 12-24. If you are looking for really wide angle, so that you can take dramatic landscapes with big looming flowers, rocks, etc. in the foreground, then you really need 12mm or wider on your 300D with its 1.6 crop factor sensor.
Telephoto. If 300mm is what you need, then you do have choices. The Sigma 70-300 DL should NOT be on your list. It is Sigma's cheap cheap lens. What you want is the APO version. It's a couple hundred dollars. Again, what are you trying to do with that lens? 70-300mm is a VERY popular first telephoto zoom (probably because of price), but 300mm may not be enough. If you want to shoot wildlife, especially birds, you never have enough telephoto, and take it from me, 300mm will only get you started. I have the APO version, and it is pretty sharp. The macro feature (works only above 200mm) makes it a good butterfly and other big bugs lens. Too short for most birds, though. Focus is slow and noisy, and the outer lens barrel turns to focus--just like the 18-55mm kit lens.
Another consideration. I see you are looking for a tripod also, but are you going to mostly shoot handheld? If so, you might want to move to a Canon lens with Image Stabilization. The Canon 70-300 IS is regarded as a not very sharp lens, but if your camera is shaking, the picture is going to be no good anyway, and soft focus is still better than motion blur.
In our house, my wife is the nature photographer. She started with the Sigma 70-300 APO Macro Super II and in very short order "ran out of millimeters". She is shooting in nearby wildlife refuges and bird sancturaries. Without the gory details, let me say that she now shoots with 500mm and a 1.4x teleconverter on her 20D. (I'm a big picture guy, so I use mostly the Canon 17-85mm IS lens.) So if birds or other small animals are your objective, then save time and interim money and go right to 500mm. Yes, the Canon 500mm F4 IS lens is over $5,000, however the Sigma 50-500mm zoom is very sharp, especially stopped down to F8 at 500mm and it works with the standard (not "Pro") Tamron 1.4x teleconvertor quite well, even retaining autofocus in most conditions. It has Sigma's HSM focus, which is really fast and quiet. No Image Stabilization, but at 500mm you should be on a very stable tripod and using a cable release anyway. You can get the Sigma 50-500 (also known as Bigma) for $800-900.
Tripods. Spend more now and save a lot later. Get a tripod that is tall enough for you to look through the camera without hunching over and without the center column extended. Also get one that goes really low. The Manfrotto you mentioned meets the low requirement, but I don't know how tall you are, so it may or may not be tall enough.
Tripod head. This is another place to not count pennies. A ball head is the most versatile--and less trouble than a pan and tilt head, all things considered. And get a bigger ball head than you need, because you NEED a bigger ball head than you think you need.
Lastly, don't discount the value of your 18-55mm kit lens. Popular Photography--or maybe it was Outdoor Photograper--did a review of that lens in a recent issue, and that lens scored quite well. Amazingly well, in fact.
