Regarding module updates, check for any that are not fully up to date. You can review the release notes logs for these from drupal.org. You'll need to review the notes for each version more recent then your installed version. We've found a variety of Drupal 7 modules that specifically list changes made to properly support PHP 7.x. It's a bit tedious but will save you a lot of stress.
Pierre G. Berryer
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