I'd buy a car that was already converted (by a reputable company) as the LPG conversion doesn't really get it's money back. You'd pay a few quid premium on a big V8 but that's about it.
I worked at a local petrol station / garage when I was a loon and they were first to get the lpg pump in the area. We used to chat to the owners and it became very apparent that some cars worked better than others with the conversion. This was 10 years ago though! I did drive a few LPG cars and they were fine. At the time the fuel consumption dropped by approximately 10% however the cost is approximately half that of petrol. Performance can drop depending on the engine but not in a night and day way. The car needs to start and run on petrol for 2-3 miles each journey so your commute has to be decent to benefit and also these miles have to be factored in to the 'break even' point - something I bet 90% of users don't do!
Anyway, what I just type might be utter rubbish nowadays but from what I've read the technology hasn't moved on that much, just the systems are perhaps slightly slicker.