The most important things to check when buying an LPG car is to make sure they have a proper invoice to cover the installation and make a note of who did the installation and check them out on the INTERNET, find out from your insurance company how old can the certification be and also will they cover you for an LPG car, some wont touch them.
Check for any smell of gas at all and if you can smell it walk away. No matter what excuse they come up with keep walking.
If you get past that and you get to drive the car check to see that the change over from LPG to gas is seamless and that there is no difference in performance, also keep smelling for gas, if you can smell it stop the car and get out as you are driving a bomb.
If the drive goes well check the installation out from the tank to the engine bay and see if you feel comfortable with what you see.
I am sorry to be such a doom and gloom merchant but I cant express strongly enough how important all these points are for your enjoyment of luxury motoring at escort MPG and your safety.
Mike