I used to do LPG conversions for a living(LPGA registered installer)
You tend to get 5-10% less mpg on gas
You need to service a little sooner especially plugs and air filter.
Much cleaner burning,emissions are very low.
Check the Lpg prices in your area as they vary depending on how many filling stations are actually in your area.
The kits themselves are easy to fit and i still have the software and link cables for the most common kits(BEDINI,LANDI RENSO,KOLTEC etc)
So the set up itself is covered.
If you are staying VAG thats fine but if you are looking to get a from factory kit installed avoid the vauxhalls as they old Necam kits were a poor design and of poor build quality and they were never actually installed AT FACTORY they were farmed out to smaller firms.
If your that way inclined and want to avoid the taxman you can buy transfer pumps from LPG BOTTLES straight to your tank.The pumps are around £100.Naughty,but only if you get caught and you can ypur fuel down to 0.50p a litre that way.
Budget £1000 for a decent set up dependant on engine whether you use a standard mixer or the more modern IGS multipoint injection systems.
You can have a vsriety of tanks but most people tend to go with the 65litre Doughnut tank that sits in your spare wheel well so you still have use of your boot space.
Remember whatever tank size you go for the actual fill capacity is only 80% to allow for expansion.