75 automatic, should I buy one?

[alanjay]

Having read your driving habits and from my experience of a 2.5 auto, expect around 28 mpg.

So if you can afford the fuel get the 2.5 auto and enjoy the kick down on those longer journeys.

As for LPG conversion, yes if you were a high mileage driver the conversion cost would be worthwhile but given your estimated mileage, I doubt if you would ever recoup the conversion costs.