I used to do LPG conversions, as i said they have to be serviced alot more frequently, if they dont then the pressure sensor can go tits up and then your into mega bucks.
Either way you still need petrol to start it, and get the coolant to 60degs.The LPG is a liquid up until -25degs the reason it has a vaporiser is to help it vapourise more efficeintly as if it doesnt vaporise before the injectors it will vapour is in the injector and blow it to pieces.
You can have a tank with out a spare wheel carrier, but you loose boot space. The tanks will only fill to 3/4 as this is a safe limit for the liquid to expand/compress Usual tank sizes for the InBoot tanks are around 50litres. But you then have to have a hole drilled in the bodywork for the filler, as it cant bemore then 50cms long. The company i worked for has the contract for Putting LPG on all the renault vans from new.
Hope this answers a few of your questions!
Dan