...glad you like it. If you copy my set-up I can't see you having any problems unless you are carrying 4 passengers and the boot full to the brim, I knew before I went for the 18's that there's no way I'd get anywhere near that extreme.
Regarding lowering the suspension, are you sure your car is standard ride height, it may already be lowered from new, you will have to check that out yourself. You can use my pics. as a guide, fit the 18's and if the car looks as though it's on stilts you will need to lower it by 35-40mm for the wheels to fill out the arches better, I can measure the gap between the tyre tread and the car's wheelarches and I get approx. 50mm or a 2" gap but remember my 156 is factory lowered. A set of lowering springs will set you back approx. £120 and to get them fitted about £80, £200 total.
If you are definitely going for 18's, 8J is the max width you need, any wider and you may have problems, fit 225/40 tyres preferably with a rubber bead protector near the rim to give you some feel through the steering wheel if you are going to touch the kerb and damage them.
To summerise, 8J x 18" wheels with 225/40 tyres and a gap of no less than 50mm between tyre and wheelarch will give you no problems even if you decide to keep the standard dampers as I have done.
As for ride quality, yes you do lose a bit of comfort, if I could put a figure on it I'd say about 15%, I've always been more into looks than practicality and when it looks that good for a 15% loss in comfort I think that's a fair trade off.
So now a question for you, what style of wheel are you into, and what colour are you going for, for more ideas go to the aftermarket wheels thread on here, it has about 7-8 pages of wheel styles to choose from with pics.
Good Luck and post up some pics. of the car when you have the job done.