In my opinion, the best is to have a number of integration teams by function (HR, Sourcing/Procurement, Finance/Accounting/Tax, Legal, Relevant Business/Marketing as needed) reporting back to a central team. Before day 1, the integration team(s) can mainly be formed by associates of the purchaser depending on the level of disclosure that was agreed with the target company while after Day 1, it is important to have mixed teams to align processes and systems (or implement the alignment that was agreed before Day 1).
