+1 search if definitely your friend on this topic.....
Big drop, go with Coil overs. Most user experience on iS is with BC coilover's, a couple members have Megan's. CO'as will go pretty much go as low as your suspension will allow. Cost is around 1K for the CO's plus 2-300 for the install and adjustment. Or you can do springs - Two main names - H&R or Eibach. Do a search for a visual drop reference on lowering springs. My H&R's cost 252.00 and the install was 450.00 Eibachs are comparable in price. H&R's are a slightly softer ride, Eibach's are slightly stiffer and tend to compress/drop slightly over time.