Is it really neccessary to run a PPC campaign for a brand new site?

I am getting a website up and running in the next week or so and we want to start SEOing our site right away. The problem is that our products are seasonal(summer months). There is still traffic now, but do you think that this traffic may be different traffic then summer months?
Anyone have any suggestions on which to run, or how important it is to PPC in the begining? What I have gathered is that in order to find keywords that will convert, we need to run a PPC campaign. I have over a thousand keywords and is there a certain formula for selection which keywords to test?
Thanks everyone. Please help!

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