A wise decision. CNG works well on bigger engines.
get it installed. choose lighter 60kg cylinders by Faber. m sure u ll get an average of 120 on high ways.
choose components carefully. u can choose either landi renzo or gaspetro kits (the regulator), the later is equally reliable and somewhat cheaper. get it installed from a nice place. n do install a lambda system. sequential injection system would be costly but give u better mileage n ll prove to be more cheaper in the long term but conversion is a risk.
from my experience get a manual switch, one that has three positions instead of a touch system one.
when installed convert only to CNG when ur engine is sufficiently warm. do not go for tappet adjustments by CNG walas. n dont let them fool around wid ur car. the kit shud work well n wouldnt require any tunning. if u r facing any problem get the regulator serviced. take ur car after every 10k km to toyota S3 shops n follow their service guidelines.