I've been reading this post carefully and thought i should give my tuppence worth of info that may help your decision.
I used to own a Fiat Brava 1.6 16v until the new year when the poor thing was written off by a combination of cambelt going and a truck taking the whole drivers side off.
Anyway i had it converted to run on LPG and had a huge 90ltr tank in the boot as i was doing a fair few miles too and from Glasgow.
I worked it out that with the price of the fuel and the cost of the system it paid for itself in around the 14 months mark.
As someone above mentioned it does prolong engine life as the oil stays that golden colour for much longer.
I used to change my oil as regularly as ever but instead of the black oil i used to take out it looked like it did when i first put it in the engine.
Petrol drops carbon deposits and other stuff into the oil and coats the engine internalls in carbon too, LPG does not have these so no inclusions it's also very very slightly abrasive so tends to clean the carbon off a used engine over time.
As for starting i don't know about other countrys practices but my system and others i've looked at recently (yes i'm having my 156 converted in a couple of months) all start on petrol and around the 1500 rpm mark the system automatically switches to the LPG.
The last quote i've had was for a new sequential system to be fitted and it was £1095 so not prohibitably expensive i don't think and will soon pay for itself.
A torodial tank (in the spare wheel recess) can as has been mentioned be a bugger if you have a puncture but today most cars carry slimmer space saving spare wheels that can still sit above the tank without losing too much space in the boot.
The tank is an option as other configurations of tank are avalable that would leave your spare wheel alone just ask the company you get a quote from for advice.
And the fuel cap on the outside of the car does not need to be places beside the original filler cap i used to have mine really low down on the rear of the bummper where it was hard to see, i know someone that has his next to the tow bar he had fitted and it's almost invisible unless you look hard enough.
As was also said above the LPG does give slightly less MPG due to the fact that it is a lower octane than petrol though LPG is mixed with butane to boost it higher giving in my opinion just as good a performance as petrol.
In the five years i had my Brava it never failed to impress me with it's take off from the lights and the top end on motorways came smoothly and quickly and surprisingly quieter than it did on petrol.
My best advice to anyone considering having a conversion done would be to talk to others that have had it done and weight up all the info you get from them also talk to a few different fitters and find the system best suited to your car and not listen to the doom merchants that tell you "it's wrong and you will regret doing it"
as most of the time they have never had a car converted.
Everyone in my family and friends said to me "don't ever buy an Italian car there crap and always breakdown"
after ignoring them i wouldn't buy a car now if it wasn't Italian
Get plenty of advice and then make up your own mind.