you need to talk to these guys
http://www.tinleytech.co.uk/index.html
very good and helpfull guys, have a good rep in landrover forums.
pre heater will meen you will get onto lpg faster on cold days, don't know if it is worth it though.
tanks, you can fit a 50/55 litre one in the wheel well, you could still have the rear seats in but the floor wouldn't sit down level or you could go for a upright one fixed onto one side, many be able to get a bigger one then.