Would it not be a more effective use of time to use the temp to offload the work of a current employee that already has already familiarity with the application/code?

As you are leery of just removing the temp tables as the scope of the candidates requirements get the full potential scope on paper. It will probably help you get a better picture on your evaluation criteria.

I don't think you are gambling, but as you are planning a complete rewrite (and you are apparently currently under-staffed) you might find this project as a time to look at options to hire and this project as a weeding ground for a potential new-hire.