hi, my dad does lpg conversions for a living, i used to do them with him until i started my new job. in a nutshell:
DOWNSIDES: where will you put the tank? ur only options are in place of the spare wheel, or mounted in the boot.
depending on personal preferance, where do you want to put the filler? (WARNING: a 2-3 inch round whole will be drilled into your bodywork!)
BENIFITS: LPG is on average half the forecourt cost of petrol.
you can re-map the timing on a lap top (i have the software of every single LPG system on the market) if you know what ur doing, just like dreamscience etc)
there are a few more finer points too. how many miles do you do per year? for example, if it costs £1200-1400 to convert it, taking into account the money you save at the pump, how long will it take you for you to get your money back?
any further questions, please just ask!