I like the idea of a contract for this person. Then your company shows a commitment, but does not have the same financial implications as you would if you went out and hired this person, and then later eliminated the position if it didn't work out.
I bet if you post on the Cove, you'll get hits for people willing to take on that contract!
There are some considerable merits in hiring an outside auditor on occasions provided, of course, one hires the right level of experience and expertise consistent with the audit objectives set by top management. It also, of course, depends on the overall strategic direction in which that top management wishes to take the company.
If, though, one merely seeks a "compliance auditor" type, I am somewhat skeptical of the value and would suggest the company sticks to using its own internal people, whatever status of full or part time management decides. That, of course, depends on the size of the firm/ site in question.
It is only when management truly wants the benefit of new views and ideas and especially the performance of a value assessment that a well-experienced outsider should be hired. And, in making that last remark, I do mean a genuine value assessment and not the nonsense currently being served up to unsuspecting organizations under the guise of value assessments but which are only the same old wine in new bottles. Caveat emptor.
To return to the original question, though, one can only advise based on the size of the company. A large company or site (say > 200) might justify the retention of a full time auditor. Smaller companies cannot. And the large company could also supplement that auditor with a number of specialists in an auditor pool. As many Covers will be aware, I wrote that, in my experience, the optimum size of the pool is around 8% of payroll, drawn company wide and company high. (See "Management Audits" 3rd edition ISBN 0951173901, page 13.)
For small companies, there are considerable merits in their hiring a contract person to perform their regular systems and compliance audits: even in engaging such a person to act as the Q Manager.
But, the more eyes and experience that can be brought to bear - the better. Those eyes must maintain currency in knowledge to work to best effect: continuous professional development is essential. Otherwise they do tend to become stale. If they are stale, hiring in a contract person to lead an audit or two can sometimes revitalize them because of the new/ different approach that attends that hiring and the outside knowledge that (hopefully) comes as well.
In developing a set of pros and cons, the overriding judgment must be based principally on these following matters: the knowledge and experience that each of the options considered would bring to bear; the strategic goals of top management; the nature of the market place in which the organization operates. For the savvy firm, budget is of little matter for, as someone once wisely remarked, "if you think knowledge is expensive, try ignorance".
Finally, qualitygoddess is quite correct, if you post an advert on the Cove for an outside auditor you will "get hits", indeed, you may well be inundated with offers. There will be wheat and chaff in the overall response. And, as was recently posted in another thread look for experience; experience; experience.