In my view some elements would be industry specific. For example, if it involves possible land contamination issues then an external DD advisor would definitely be necessary. In other respects, I think the typical parts of areas requiring external advisors would be legal and perhaps financial/tax. Even with an in-house team, my experience has been that external consultants are still employed – partly because the internal resources may be taxed for time and many not have all the expertise needed, but also unfortunately because many companies or investors need some aspect of the deal rubber stamped by a third party to reduce exposure to potential lawsuits. It of course varies also with the type and size of organization involved.