“Once ready” they’ll save (somewhat) on licenses, what about paying for it during the years it will take to build it, while it’s not ready?

When any random company makes a Build vs. Buy decision the question is “is this a core competency?” Most companies use a package from MS or GOOG because it’s unlikely that they’ll be so good at productivity software that it’s (A) worth distracting themselves from their actual job and (B) good enough. The same caveats apply here.