If the above comments still don't put you off, I'd advise looking extensively at the paperwork. I know two guys who've completed/are completing 170 conversions (one in progress includes a power works supercharger kit). Not everyone is ragging them though, and you'll be able to tell instantly what the conversion was made for.
The guy who's done the conversion, uses his for track days and the like. Never uses as a daily driver. The guy with the supercharger kit going on, is completing the work himself over a long period of time as a weekend car. Neither are intending to rag.
The 170 engine is very strong, and the gearbox is one of the best Ford have ever used (it's a 6 speed Getrag - even the ST/RS don't have this box). However, I'd still be tempted to get a proper 170 in your position. I had one, and it was good fun.
Not the quickest to 60mph from standing, but from 40 upwards in-gear, it's a lot quicker than it gets credit for. It can hit 72 mph in second gear and revs to 7250rpm