Dean,
The 17" alloy wheels - referred to as "Cup" wheels were an option (M403) on both C2 and C4 but they were 7J-ET55(F) and 8J-ET52(R). These wheels are often referred to as "Cup 1" wheels and are no longer manufactured. They are a little like chickens teeth to get hold of on the second hand market too. You will not need spacers with these wheels.
HOWEVER - there are lots of Cup 1 replicas about which can be a different size as well as the Porsche originals supplied for the Turbo-look cars and the RS in 7.5J-ET55 and 9J-ET55 sizes. The RS versions were magnesium based and are like chickens teeth with gold caps to get!!!
You can't decide whether you need spacers until you know the dimensions of the wheels you intend to use. And THAT will depend on what you can find!!
Good luck in your search.
Regards
Dave