An existing Drupal 8 webpage consists individual non-drupal moduls, which are not supported by the new Drupal Core.
Such Moduls need to adjust / to repair.
New Drupal Core need to install.
Several extention works.
A strong professional Team is requested,
no "lonely wolfs",
"you must belive me"
"all your mistakes by you ONLY"
and others specials i found already in freelancer or other places.
A clearly work, professional provided & processed should be the only way.
About me - i am not the professional Drupal Programmer, and i´ll ask a lot.
I´m Professional in another job - but not stupid ready for cheating by an IT-Guy.
Any Question, let me know.
Do not quote without reading above.