Optical "Zoom" is really just a function of the lens itself and is reported for point and shoot cameras because they can't change lenses. Zoom is just the ratio of the longest 'focal length' (in 'mm') of the lens compared to the shortest. For example, the particular lens that comes with this camera has a range of 18-135mm so its zoom is 135/8 = 7.5X. With a DSLR, you can add more lenses to your kit to expand your overall zoom range like, for example, an 8-16mm ultra wide lens (for extremely wide views of sprawling landscapes) or a 70-300mm telephoto (for close-up views of faraway subjects) or even a 100-400mm lens giving you much more range than your current camera but at a much higher overall cost. With a DSLR, you can also purchase a "super zoom" lens with large ranges such as the Tamron 18-270mm givng you a zoom of 15X though the design of a lens with this kind of ranges generally suffers from compromises in terms of performance.
Yes. Any lens compatible with the t4i will be compatible with the t3i. They have the same lens mount. They are basically the same camera. The main difference is that the t4i has a touch screen and the t3i doesn't.
There are some issues and drawbacks to using a teleconverter/tele-extender you should be aware of:
(1) You lose light - a 2X TC loses two stops of light meaning the sensor will only get 1/4 the amount of light which can and will cause auto-focus problems unless you have a fast lens (e.g, f/2.8). A 1.4X TC only loses 1/2 the amount of light.
(2) Image quality can suffer to a greater or lesser extent depending on the TC/lens combination, lens quality and whether you're using a 1.4X or a 2X TC.
(3) There are no teleconverters to my knowledge which can mount EF-S lenses
(4) Lens makers design their teleconverters to only work with their own high-end lenses. These are generally incompatible with other lenses.
(5) Teleconverters are designed for telephoto lenses. They may not work well with hyperzoom lenses or other lenses wider than 100mm or so.
(6) You might be better off cropping your images in a photo editor in some cases.
The two best generic teleconverters I know of are the Tamron PRO and Kenko PRO. These seemed to work with most (non-EF-S) lenses but the camera would not autofocus at all without a faster lens.
18-135 is a better lens. you won't be sorry. 55-250 is ok but you'd do better with a better zoom later. 32 gb card is over kill. most people could get by with a cheaper 4 or 8 for recreational use. buy one for a couple bucks at walgreen's. don't shop at wal-mart, please.