How difficult and approximately costly is it to convert a traditional WordPress site into headless so that we can use a or have a react front end?

I do not know about costs. But you have two cases:

1. To pay someone who knows React or Vue or JS and more to do the project
2. To read and do it by yourself (you are going to pay for the education and you will need time)

WordPress is used as haedless CMS, but there are more solutions like Drupal and more. Just read about alternatives.Try to find similar project to see what is the history, the plus and minus. In general the easier is Vue JS.