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I wouldn't suggest buy the most powerful CPU, graphics, etc... That really isn't taking the right path, nor a good suggestion for a person that just needs a computer for word processing.
You want to do your research, figure out what your going to use the machine for. If your building a good gaming PC, or a PC for video editing, and such then yes I would recommend the top of the line stuff. But if you're just using it for work (like word processing or internet use), then you can scale down the CPU, get an average video card. The processor is what really makes the computer expensive, remember that. If you shoot for the top of the line processor, then be expecting to pay a lot of money!
Good places to buy parts:
As for tools, you'll need the basics just to get the thing together. But the most important part with building machines is; 1) what you're going to use it for, 2) how much money you're willing to lay down.