Thanks for the good wishes and input. The reason for a handheld is that windscreen mounts are unacceptable for the race, as is anything that might get in the path of the airbag. I'll be handing my car back when its lease finishes 3 months later (I hope the manufacturer isn't reading this) and so I don't want to start bolting stuff into the dashboard.
So far, I'm $980 into this in entry fees alone, and I've had to upgrade my race helmet and gloves too, so I'm thinking I might even skip buying a GPS this time around and go with the "traditional" stopwatch and distance chart instead. Then, once I've seen the course properly, I can get a GPS for the September run.