I will ask the guy about the range rover.
What I have heard from a BMW dealer, a guy had his 4.8 cenverted in Italy for abt 2000 Eur.
He put a 90 liter tank in the spare wheel space, which he can fill up to 75 liters (20% must be empty).
The thing is that the car's mileage, when using LPG, decreases by 20%, but the price of LPG (here in Croatia) is 52% cheaper than regular gasoline.
What you get in the end is 30% less on your fuel cost, cleaner engine, less CO2. You only lose top speed whereas you do not lose much on the HP.