You should consider using an ISO 27001 consultant if your employees do not have enough time to deal with the implementation, if you have a very short deadline, and if you have the budget for such an engagement. A consultant can help by defining implementation steps, conducting interviews and analysis, writing drafts of documents, and training employees. However, if your employees have some time for the project and your budget is not too high, you might consider implementing the standard with your own people using external help.