Congratulations on your new site for Sacred Roots. You did a really nice job for them, I am sure they are pleased.
Redirects from old links to new ones is smart. Do you need help setting them up?
Here is a list of files and directories that come with WordPress, so you need to keep all these:
Of course, before deleting anything, make a copy of everything in your WordPress directory. Then, as long as you keep the files and directories listed above, you can delete the html files used by the old site.
Nothing should go wrong, but if something does go wrong, just copy your backup back into the directory, returning it to the state it was in prior to any deletions.