Here's my issue with writing out your content first...

The purpose of buying SBI! is that it will help you research the topic/idea so you make sure you have chosen the most "profitable" topic based on keyword searches and supply. What if you write out your content first and then buy SBI! only to discover not enough people are searching for your topic. Now what? So now you realize you may need to choose a different topic.

I guess that's my only issue with building your site first if you plan on buying SBI. That would be fine if you were just going to use a $5 host that doesn't give you the analysis tools. But the purpose of SBI is that it helps you do the research so you know that you're choosing an "in demand" topic.

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Just call me Max.