Hi there !
18" will fit, just check that you get the right offset, otherwise they will scrub in the arches. I have 7.5*18" Offset 42 with 225/40-18 tires and there are no problems, and this is a 5d..the coupé has a bit bigger arches in the back..
For the lowering there are a few nice combos to choose from..I use adjustable yellow Konis and H&R springs and this is a great combo !! Then there are Eibach springs and Bilstein shocks that also are good !
As far as bodykits go, there are not that many kits out there for Saabs..As Pyronerd said Abbott has some bodyparts..Then there are a few companies in Sweden that makes kits but someone said that they dont want to ship the kits too far since they might brake during the shipping..Don't know..
But
Johansson Motor Sport makes one nice kit and then if you want something a bit more extreme Real Car has a nice kit..You can find it
Here
Then there is always the Viggen/Aero kit witch also is nice and you can get it directly from Saab, the only downside with that one is the price..