Hi there, @pitchplan - welcome to Shopify!
So there are a couple of options here - you can of course simply keep the site you have built on Wordpress and add a Shopify “buy now” button to that website through either our Lite plan or Starter Plan. You can read more on this option here to see if this might work for you.
The other option would be to build a new online store using Shopify, and migrate everything over to Shopify. This would also mean redirecting the domain as you mentioned above, but this is very doable.
We have an in-depth guide here on migrating from Wordpress to Shopify here, it is definitely worth having a read over.