Great advice here-you are asking very good questions and doing your research will pay off. I personally spent almost a year reading and researching before I even bought my GS4 and already had a good idea of what I wanted to do. Of course, I have been modding every one of my cars for...let's just say a loooong time
I can personally attest to the vast improvement in ride, handling and appearance the Tein CS coilovers will give. I wanted to slightly lower the car both for looks and improved cornering (get rid of the "floatiness") and still maintain a decent ride. Mission accomplished! I had my Daizen sways installed at the same time for maximum benefits and saved a bit on installation costs as well. EXCELLENT COMBINATION
Even with the stock 17s and original tires, the difference was amazing! Well worth the cost IMO. I swapped out the stock tires for better ones on the 17" wheels, but recently found a nice set of 18s and the Goodyears (see sig). Again, huge difference in overall handling! Personally, I preferred the 18s over the 19 and 20s many like due to the weight penalty of the larger, heavier wheels. I am interested as much or more in performance and for me, 18s were the better choice.
Good luck with your decisions-take your time and ask lots of questions as well as searching. Post some pics when you get a chance!

 

Thanks a lot, Future. It's good to hear someone else has tried the CS, Daizen, 18s combo before that worked well. I'm sure your handling is greatly improved...how much stiffer has your ride gotten?