You don't need to purchase anything.
Your camera came with the Canon software; one of the applications it comes bundled with is EOS Utility.
EOS Utility has remote live view shooting capability. If you set your lens to autofocus, you can even focus from your computer. For precise focusing, click the checkbox titled 200%.
Be sure to turn autofocus to off on the lens when you start photographing.
EOS Utility also gives you the ability to set up an intervalometer -- x number of shots of y seconds duration with z seconds interval between each shot. Plus, an initial delay. Double your value for z if employing in camera noise reduction.
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