The consultant should be certified to perform the work they will be providing. This guarantees the manufacturer of the hardware or software will support them in the event of implementation issues. It also provides a framework to work within to get problems resolved quickly and efficiently. Every major software or hardware vendor has a certification process. The certification also guarantees the consultant has had a certain level of training to be competent on the platform being installed.
Business is about relationships.
You must be able to trust the consultant you use in order to open your computing environment up to them. They will have access to your data and resources. If you cant trust them, you should look for a different consultant which you will trust. The best consultants should represent your best interests and be able to be trusted the same as if they were an employee of your company. Consultants should be looked at as a strategic business partner.