well as i am driving cultus for the last 5 years what i feel that its a nice economical car with weak interior and some weak parts starts making noise/depreciated if u do not take good care of ur car...
but i recommend u VITZ ... u will feel that u r driving a good luxuary car ... but its bit expensive spare parts wise ... but it has power, looks and other stuff way better than cultus...
and regarding cng in vitz .. my cousin has 2003 model and he is enjoying his ride on cng for the last few months so far no issue... but not sure about 2005 model ... shope some expert will reply u