Does anyone have any recommendations on how to get a head start on their SEO when building a new ecom store?
1 Idea I have is maybe buying a word press blog site, and moving everything, domain, content, etc. to Shopify.
Shopify stores can rank just fine. Don't let a bad URL structure convince you to make an expensive move you don't need. You likely need to do a thorough job of SEO which includes a technical audit, keywords, internal link plan, external links, et cetera. Just making a change may cost you more ranks even with 301 redirects then you expect.