The system fitted there is a dream 3000. quite easy to install tbh. but parts alone come to 1.6k.
as above the multi-point injection systems are deffo the better systems, and you also loose mpg but the cost of gas is alot cheaper.
You can also change from petrol-gas when ever you want. but it needs petrol to start until its warmed up.
the tanks are usually 25l tanks if its a 'donut' which means its in the spare wheel well, it also will only ever fill up to 75% of the capacity, due to gas expansion etc.