Have a look at the Beta 10 or Beta BZ482 - Same engine but first is restrcited to 10HP. Good company to deal with and aftersales is pretty good. £3k for engine inc all sterngear etc and about £1k to fit. All very rough prices!
the smallest yanmar 1gm will cost about £2200 boat show price in england, plus about 1-1.5 k to fit unless diy. very reliable engine. Have you considered the Dolphin engine? petrol driven, very reliable, light, quiet, and no vibration. No idea about outlets in Oz, though.
try the new ten horse Nanni - made to fit in the same space as a 1gm10 Yanmar, but twin cylinder, and fresh water cooled. A great little motor! take a look here for more info..