let me tell u first that i havent driven a new city on cng or havent even talked to an owner of it whos running it on cng.these replies are based on technical facts n sum logical reasons.
1) the result wud be fine.nice economy n performance. cuz the engine is basically the same L series engine ,with just vtec added.
2) Landi renzo kit is the best, for cylinder, i ve heard EKC isnt a good one, though my car has it.but better go for any other, may be FABER,INFLEX or sum other brand.
3) Milage should be good enough, may be 130km/cyl in local driving.though it depends on the driving styles etc.
4) yes there wud certainly be sum strain on the suspension as u wud be putting extra load on it all the time.there r 2 basic things u can do for it.either go for specially made CNG-purpose springs available in the market, or else u can out packing/spacer under the stock springs.this wud lift up the car to its original height but wud make the suspension lil bit harder. cant say if it wud reduce strain or not, but im using this one for about 3 yrs ( 1lakh plus kms) and the rear/front suspension is still perfect.n i love hard suspensions so its fine for me.
hope that helps a little bit.