I own a Canon 550d and wanted to buy telephoto zoom lens is it essential to buy high end body like 7d or 5d mark iii?
No. All EF and EF-S lenses work on your camera. As a matter of fact, you get more reach with a cropped-sensor like yours or that of the 7D than you would with a full-frame model like a 5D Mark III.
So if you are looking for to get a long reach, you will save money, size and weight by staying with a cropped-sensor body. For example, a 200mm lens on yours gives the same field of view as a 320mm on a full-frame.
No, absolutely not. A telephoto lens will work just fine with your existing camera.
Just as there are high-end and lower-model camera bodies, there are different levels of lens as well, and this is particularly true for large focal length (commonly called "telephoto") lenses. If you spend $10,000 on a lens (easily done!), you probably want a top-level body to go with it. But you can also get an entry-level telephoto zoom for under $200 (like this one).
No, not at all. Although a heavy, well-built pro body will balance a heavy telephoto lens better than an entry-level body, and obviously the IQ will be better on the pro-body compared to the entry-level body.
No, not at all, as others have said.
Now, so we can be more helpful, what do you want to shoot? And have you looked at any lenses already?
For instance, I just spent $80 on craigslist and got a 75-300 F4-5.6 zoom. Its not a great lens, but it works fine in bright light. And it was cheap.