I would suggest you to go for Cultus if and only if you know how to maintain a car. If you do not care for your car, then go for a brand new one as most probably it will not give you issues for the first couple of years.
Secondly, Cultus is obviously much luxury, powerful, comfortable and good looking as compared to Mehran. It's A/C is also better than that of Mehran's. You can take it to long routes as it would be more reliable.
As far as the "kharcha" is concerned; for how much will you get a good A/C for Mehran? Obviously it would also cost you something if not as much as CNG kit for Cultus. The issue that you have mentioned regarding the suspension of Cultus is with the boot or commonly known as "godda" as it makes "tak tak" sound while turning only. Well, that is not a major problem and will not cost much money and time.
Also compare the overall expense that you may have after buying either of the cars. You might go for new tyres for Mehran, seat covers, foot mats, most importantly registration etc. How much do you think you would be spending on Cultus? As the engine has been used on petrol, I am hopeful it wont have any issue. Check the tyres of Cultus, as it is also important. As it is said, "kurri pairan to te gaddi tairan to".
CNG would be the only major expense but consider the following equality:
Cost of CNG = Cost of A/C + Cost of Registration
You may be satisfied with a new Mehran, but you wont be satisfied by its A/C cooling, performance and comfort.
Verdict: It you think you can maintain cars, black cars, then go for Cultus after doing possible bargain.