Personally I would steer clear of LPG on all models, rarely do the sums truly add up. However you need to be clear on what you are looking to buy. A Range Rover "Vogue" is a trim level, not a model. You are looking at Range Rover L322's. The "Supercharged" is also actually a trim model, although most of the time its the same as the Vogue SE trim level with a few bits of S/C trim and rear lights etc.
If it has a Supercharged engine of any size its an AJV8 or Jaguar engine.
MY02 to MY05 the only petrol engine was a BWW 4.4 M62 engine. Of all models this seems the most accepting of the devils gas.
MY06 onwards also had the option (confusingly) of a 4.4 AJV8 NA engine, more powerful than the BMW 4.4 M62 engine but apparently does not take gas as well as the M62. The easiest way to tell from outside is the headlights will be the later type on an AJV8 but many older M62 engind cars have face lifts nailed onto them so the best way is to get an under bonnet picture and then you will know.
Despite the M62 being better at taking gas, it comes with the fairly poor 5 speed gearbox and the older navigation system etc older dash cluster and unless its very cheap these days its probably best avoided.
Generally the sweet spot for reliability is considered be the MY07 onwards supercharged machines, generally the least to go wrong and MY07 brought in terrain response and the updated interior which helps quality and brings them a little more up to date.