1: Yes, maintenance of Passo is expensive as compared to Cultus, and parts are surely expensive as compared to Cultus since Cultus is an economical suzuki car and Passo is an small luxury jdm.
2: Yes, you can easily find 2007-08 unregistered car.
3: Yes, Passo is surely a reliable Toyota car and also because it's JDM BUT note that reliability depends on the condition of the car you buy, in other words if it's a neat car in terms of engine, body, suspensions etc then you'll make a good purchase and car is expected to be reliable.
4: I've no idea if CNG could be installed as my car is on petrol, but according to my knowledge if you install CNG in passo you might face knocking and pick issues and most importantly installing CNG kills the value of car as per my knowledge.
5: Dealers here in khalid-bin-waleed Karachi are sitting to make the fool out of people, ODO's are MOSTLY reversed and claiming that with fake auction sheets, 85 to 90 % ODO's are reversed and auction sheets are fake. I'd suggest you to go to Super high way there are showrooms nearby bus-stop, dealers are phathans and there cars are in mostly genuine forms and very low chances of reversed ODOs with way better prices than khalid-bin-waleed. I purchased my 2008 car from there, I'm lovin' it MashaAllah se no issues at all.