I had the same party on my first three saves. Each save I chose the Thief for the blank spot. There will be fights where the wisest decision will be to run. I can't recall off the top of my head anytime my thieves have ever failed to run from monsters that are programmed to be....'runnable'.

It is ironic saying this, but my other alternate suggestion would be the BlackBelt (Monk). He needs little or no equipment to progress in power, saving you quite a bit of G.