Lots of cheap brands of inflatables around these days.
Zodiac meets the market and still maintains quality and back up.
The Queensland distributors web site here:-
marinesafe.com.au/and great people to deal with too.
I would recommend the Cadet S C260s (ply floor and inflatable keel) which I believe is rated for up to 8hp engines. It will go well with a 4, fly with a 6 and hurtle with an 8.
A Mariner/Mercury 5hp would be a good choice being a simple robust yet powerful single cylinder engine with an integral tank with capability of being fed from an external fuel tank.
It should get the boat planing two up and will give good speed three or four up.
The later model 2.6m Zodiacs with the rounded tube ends have the same internal room as the earlier 3.1m models.
The ply floor models are heavier than the air floor boats but I think they can take a bit more punishment.
Keep a good eye on Gumtree/Ebay/Trading Post and you will probably come with the ideal boat/motor package for less than $1,500.