Hey,
Nice looking wheels. They look like Alt Miros. A couple of guys here have them.
Spacers are required for some wheels to keep the tires from hitting the shocks. Other times, owners use spacers to bring the wheels out to the edges of the fenderwells for a more aggressive look. In either case, longer lug bolts are necessary to ensure your safety.
As far as your ride goes, anytime you go to a smaller sidewall, you'll get some deterioration in ride quality. my car had 18s on it when I bought it and I moved up to 19s. I noticed very little difference in the ride. Others claim a marked degradation in their ride.
Check with Carlos at
http://986forum.com/forums/wheeldynamics.net . He stocks the Alt Miros and many other wheels at competitive prices. Best of all, he knows exactly what will fit and what won't.