Migrating and working behind the scenes

I’m new on the forum, so if I have posted this in the wrong place, let me know.

I have a goal of migrating our static HTML site to WordPress. Probably the best scenario, since the site has hundreds of pages, is to set up the new WP site somewhere else, test it, finish it, test it some more, and then let it go live and replace the old site.

Would the best way to do this be: a) temporarily set up the new WP site in a subdirectory that will later be promoted? Or, b) set up the new WP site in a related domain name? (Everything is on sampledomainname.org, but we also own sampledomainname.com and sampledomainname.net, which for the time being forward to .org.) Or c) is there another option I should be considering, perhaps a way to make the site a hybrid while it’s under construction?

I’ve checked out some of the migration threads in the forums, but I’m still unclear about the best way to proceed.

Thanks in advance.