I have owned and repaired such cars - they are quite robust and very long life, hence expect to replace parts on recommended intervals. Parts are not that expensive but some jobs require special tools and techniques or you may ruin an item. Benz' are not that difficult to repair, they are just built very differently.
About the W202 - its very nicely made, and very very long life, the engine will accept CNG like a duck to water but proper recommendation should be followed there is no room for jugaar
I noted a point above of fuel pressure, It makes me quite sad to see MOST mercedes independant mechanics in Pakistan tackle a job without proper diagnosis, The worst is when they think that they can adjust the Kjetronic by feel