I have looked into LPG conversions over the past 10 year for various cars i hae owed.
Right now it’s for a 530i BMW that my wife drives.
Best price i could get was £1600+VAT fitted. Go for a spare wheel tank and you will get about 40l of fuel, but at least it’s out the way if you need boot space (obviously you will have no spare wheel and need a can of foam or AA membership at least)
Go for the sausage tank and you are looking at about 60l of fuel capacity, bigger tank but less boot space.
For me we have a new baby so boot space was important,
problem with the smaller tank is that i would have a range of about 180 miles tops (not great)
Bear in mind that you will get about 5-10% less mpg using LPG (less energy in the fuel over petrol)
We only do about 8k miles a year so it was not looking very cost effective, to be honest its the small range that puts me off the most, and the closest LPG supplier to be is 7 miles away.
Something to think about.