I suggest a Commodore as it sells much better than the Falcon and the Falcon may be cancelled soon which may affect parts and resale values.
LPG on a new Commodore costs $2500 but you get $2000 back in a rebate so it will pay for itself in no time.
"Using an 84.4-litre aircraft-grade aluminium fuel tank, the LPG Commodore Omega delivers 710km of combined city and highway driving on a single tank (11.8L/100km for Omega and Berlina – 12.3L/100km for other sedan and Sportwagon variants – 12.4L/100km for Utes). That’s more than the Toyota Aurion ATX (707km), Mazda3 (696km) and even the Ford Falcon XT (687 km). It’s also more fuel efficient than the Falcon EcoLPI. Better still, based on the average 20,000km a year cycle, the LPG Commodore costs around $1227 in fuel per year, which is very similar to the more powerful Falcon EcoLPi ($1279) but significantly cheaper than the Toyota Hybrid Camry ($1644) and Corolla ($2028)."
From here http://www.caradvice.com.au/159889/holden-lpg-commodore-review/
Definitely go the big car. I used to drive 3 hours to uni first in a 90 Excel and then a 91 Commodore wagon. If anything the Commodore had better fuel economy on the highway. Both cars were petrol. The difference in comfort and cargo area is huge.
Do you have a budget in mind?