I would be very careful about buying an LPG system second hand. Yes, it can work out cheaper, but bear in mind that there are a lot of parts you will still require. All the hoses and pipes, along with quite a few of the fittings, will not be safe to re-use IMHO, so you will have to buy new.
The loom may well have been cut to suit the previous car, so this will need to be reworked when you put it into a new car.
You may not have, or be able to get hold of, a wiring diagram to work out how to splice the loom into the car's electrics.
You may well find that tuning software is available to professional fitters only, so you can't tune the installation when it's finished.
If you can't see the original vehicle running on LPG before purchase, the kit may have problems that will only make it more difficult to get up and running on gas.
I would only recommend going that route if you have converted a couple of cars in the past and know your way around an LPG setup very well. Even then, it would have to be very cheap to be worth the potential extra aggro, IMHO.