From your standpoint is there really going to be a big difference between what would make a good mystery shopper and what would make a good employee in general? Those of us that operate as successful Independent Contractors need some skills and motivation not as necessary for an employee. Meanwhile, some more specific skills could be dependent on your company, the scope of your mystery shops and the level of detail you desire.
If your company is paying an hourly wage and expenses, it would also be a safe guess the company will have solid expectations for how many assignments will be completed and how many miles will be traveled. We ICs usually make those determinations for ourselves and thrive on the flexibility. You may need people who function better with more structure.
Equal rights for others does not mean fewer rights for you. It's not pie.
"I prefer someone who burns the flag and then wraps themselves up in the Constitution over someone who burns the Constitution and then wraps themselves up in the flag." -Molly Ivins
Never try to teach a pig to sing. It's a waste of your time and it really annoys the pig.