Would there be a need for external advice in case a company implements an integrated system of management?

Yes of course, the successful companies want to learn and move on all the time.

Extrenal advice has importance because you can think for alternate. Its totly depends on management teem that decision should taken or not but if we even not listen the advice or just follow to system then may be miss some important opportunity.

Yes, Its always better to seek external support and advise from the issuing authorities of certification and auditing. Its recommended to ask support from the same company which actually re-certify your certification as to how to integrate different management systems.