Here is mine- I think the trade off between looks and road behaviour favours 17"s - I am sure others will disagree but I would rather have the slightly less crashy ride on english roads although I like the look of 18"s and would never run down those for whom that is decisive. Light wheels reduce unsprung weight and should therefore
help the roadholding by making it easier for the dampers and springs in keeping the tyres in best contact with the road. So your plan makes sense if you think the looks are OK.
Mine is lowered with the pro-kit Eibachs (30mm nominal)- Sportlines are a bit
lower still. I can vouch for the Pro-kit as a smooth and improving spring for the Fr- no clearance issues. Others have posted favourably about the Sportlines.