I don't think there's a right or wrong answer - or even necessarily a best answer. You could do it either way, but different developers might approach the project very differently.
Personally, if I undertake a project to turn any static site into a WordPress site, I would build a child theme for my favourite theme framework. If the site has been prototyped as a static/non-wp version, I would use that as a visual mockup and probably wouldn't use much if any of the markup & css from that version. The developer you use may prefer approach it in the opposite way though.