I hope you are getting the gearbox for free as the cost of petrol will cancel out any savings on the gearbox.
When I went to Sydney recently I ended up using 7 tanks of fuel on the trip = $550
As you are just doing a road trip count on about 5+ tanks of fuel = $400+.
Have a look at this on ebay.
http://www.ebay.com.au/itm/wrecking-...#ht_481wt_1417