Would you be keeping the 318is?
Since the approx. fuel costs are the same per mile traveled, I would say that if you can trade in your car and pay 1000 out of pocket, then go diesel. Then you would have the option to go grease in the future, which would mean possibly free fuel.
Otherwise fit the LPG to your car. You would have to factor in diesel repairs, and whether they would cost more into your analysis, also.
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