i would definitely go with diesel now, i have been running lpg since about 2003 on a volvo, i fitted my own system then and it was at the time 35 pence per litre, i am still running a 3ltr volvo now and the price has just about doubled, i plan on selling the volvo in april and putting my 1963 landy on the road, as you can run this on part cooking oil.
I dont think it is worth investing money into lpg anymore