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Wow you've asked some crucial, important, interesting and hypothetical questions there 
It sounds like your getting or have a firm grasp of what seo is and want to go about experimenting with it and gauging its results upon researched criteria.
My humble opinion would also need to know a few more variables, how long has your current site been running (if you added it as a sub-directory) and is it relevant to the sub-directory content. And is the domain you want to buy an exact match to the keywords your aiming for? - Along with a few other thoughts.....
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