There are certain parts or options that can make a car worse to DD on their own and not from a correlation of HP. Like turbos and clutches being the two big ones. Using the wife/gf standard for DD, I know that my girl cant drive a geo with a 200k mile clutch, let alone a new 6 puck. But shes more than comfortable with a stalled auto.
With a turbo, youll have to warm the car up before getting on it, and let it cool down for a minute or two before you shut it down. Also chances are that you will eventually get a tuning device with some sort of in-car adjustability. Imagine telling the Gf not to push the Big red "scramble boost" button.
With that said I would draw the line at around 500 or less. At that HP, a driveable clutch CAN be had. You can do it with a street-able turbo, something that wont have you looking at guages everyother heartbeat, can do it on pump gas, and you might even be able to get a little boost when your driving stoplight to stoplight.