It certainly would not hurt if some of the Justices had either had experience as workers outside the political arena in addition to their training in the law, but I do think a law degree or its equivalent (e.g., having served as a legislator for a few terms) would be a requirement.
At the very least, a judge needs to be able to read and understand the Constitution, court arguments and decisions, etc.

As for your previous statement about "maintaing balance," Rig, surely you realize that having even 5 white males on the court is not "balance" in terms of representing the population.

Obviously in a group of nine, we couldn't have proportional representation by gender, religion, ethnicity, etc. But if we replaced 3 more of the white male Christians with people who were Jewish, female, Asian-Americans, and/or others of minority background, we'd come closer to that kind of "balance."