Premium is a one off cost which I guess will give me everything I need or do I need to customise it after.

A Premium Theme purchase is a one time purchase for the lifetime of the blog while free hosted here at WordPress.com. premium themes cannot be exported and used on a WordPress.org theme if you move the blog content into a WordPress.ORG install in the future.
http://en.support.wordpress.com/themes/premium-themes/

Whether or not you will like any the off the shelf premium theme with the customizable options it has, or you will want the ability to customize it further by to purchasing custom design upgrade and undertaking editing the CSS is unknown to us Volunteers.

The custom design upgrade comes with Staff support but that support is not instantaneously delivered. Those Volunteers who provided it free of charge in a timely fashion for years are now on long term walkabout, so we remaining Volunteers don't recommend purchasing it unless you have CSS editing skills and know what you are doing.
http://en.support.wordpress.com/custom-design/
http://en.support.wordpress.com/custom-design/editing-css/
http://en.support.wordpress.com/custom-design/#frequently-asked-questions

Can't see the wood through the trees on all of the options. Would love some best practice advice please.

Thanks. John :)