David, I don't mean to pour cold water on your enthusiasm, but this forum is littered with sorry tales (mine included) of windscreen replacements that have gone wrong. My suggestion, therefore, is to make a good photographic record of how the original screen and seals fit around the roof line, down the sides and along the bottom. On completion have your photographs to hand and check that there is a smooth transition from glass-to-seal-to-roof with no roof-line step visible, Also pay attention to how well the seal fits along the bottom edge of the screen and don't accept "it's not important and they never go back like new" type excuses, because, after a few failed attempts, they can eventually get it right.
Finally, although it may not be relevant to the 997, I have heard of corrosion problems on 993s arising from paintwork damage caused by careless removal of sealant, this may not show for a number of years, because it's behind the glass, so it's not a problem unless you intend keeping the car.
BTW in Sept 07 an OPC replacement screen cost £867.