there are 3 types of NGV conversion kit in the market ranging from RM2500-RM8000
1. For carburetor car (Mixer Type)
2. For fuel injection car (Pre-mix Type)
3. For Fuel Injection car (Sequential Type)
both 2 and 3 is suitable for BMW but the 3rd one is so much better because the kit has individual NG injector for individual cylinder instead of mixing NG and air before putting the mixture into the cylinder (system 2).
In that sense, the 3rd option can control the fuel air ratio better than 1 and 2.
NGV advise to have a TAP (Timing Advance Processor) to adjust the timing because NG octane is rated at 120 while gasoline is rated at 97.
The biggest tank for NG is 80 liter which can only be fitted in vans instead of any bmw here. If you get a 60 liter you will get to fill around Rm9-12 depends on pump pressure. From that full tank you can get roughly 200km.
In europe, there are LPG vehicle where one full tank can go for more than 500km. LPG vehicle is somewhat illegal here. I dont know what's the reason.
For road tax, only 25% off when you are using NG and gasoline; if you use 100% NG then you are entitle of 50% off on your road tax like those airport limo.
If you guy is interested, you can always visit to the local NGV forum
http://www.ngvcommunity.com/