و عليكم السلام
Two types of engines come in Nissan AD 2007 new shape.
Nissan CR12DE 1240cc (which the dealers call it 1300 cc),
and Nissan HR15DE 1498cc (which the dealers call it 1500cc).
1300cc version:
I have 1300cc version. Its air intake path is very compact and due to this the installation of "Common CNG kit with venturi" is quite tricky and common mechanics just refuse to install. You need very expert and technically sound mechanics for the installation because a carefully designed venturi needs to be inserted in its air-flow path before the air-filter. I did not install this kit rather i opted for sequential injection system which i have described above in this thread. It was working good until a problem occurred in the ECU of the CNG kit as I had installed a lower grade kit that had some inherited problems in it. But if you install a good kit it will work fine. May be in some days i will be back on CNG after.
1500cc version:
This version has different engine design than the 1300cc version. I don't have this version but i have seen the engine. Its air intake path is quite simpler as compared to 1300cc version and the installation of "Common CNG kit with venturi" in it is simpler enough. I have seen two Nissan Wingroads with this kit installed. One had open loop CNG kit and one had lambda control CNG kit. The open loop had the problem of continuously on check-engine light while the one with lambda control system had no such problem. And i guess Nissan Wingroad 1500cc has the same engine. Similarly you can go for sequential injection system but it costs high.
I will advise you to buy 1500CC version. HR15DE is a bit newer technology engine compared to CR12DE. And if possible, adding just a little more to the budget you can opt for Wingroad which is very much better than AD van in terms of comfort and luxury. (Though Nissan AD has very comforatable drive and front seats but its rear seats are quite bad and i have replaced them).
And one more tip, after buying any used imported Japanese car you must change its old liquids (coolant, engine oil, transmission oil) so that you are on a safer side. Older liquids may be old enough to degrade the life of you car. And in 1500CC also try to replace the timing belt of the engine with a newer and genuine belt as it may have exceeded its life. (1300cc comes with timing chain and you don't need to change it).
Best of luck.