All depends on how many miles you do and how long you intend to keep the car.
There are also some good second hand ones with the kit already fitted...so you have to weigh up the costs.
I looked into some of the multipoint injection versions and had quotes around £1500 to £1800.
I only do 6000 miles a year and for me I decided not to bother, mines a ML350 and does up to 28mpg on a run which I'm ok with
As for size of tank, if your not too bothered about boot you can have a large tank behind rear seats, or some even replace you fuel tank with LPG and leave a smaller petrol tank as 'reserve'
My father had LPG years ago in a C200 estate and it was an early system, he had a couple of small issues at the start but then he did 100K miles in that car. The LPG conversions have come a long way since then and also more and more fuel stations sell it, especially in europe.
All depends on your miles and how long your keeping the car, I guess you don't do too many miles hence buying an ML500