If you're on a budget then don't worry about getting a 4x4 - you can still get well off the beaten track in a normal car. The Yaris we hired was fine for driving on the non-tarmac roads (though I'm glad it wasn't my suspension taking that abuse) - we followed some routes suggested by a guidebook and saw some amazing scenery that you'd miss if you only used coach tours. Be sensible though, take a mobile and check the spare tyre as I'm sure it wouldn't be hard to get a puncture, and we sometimes drove for hundreds of kms without seeing a single living thing!
Avis was cheapest when we went in late September, from the airport.
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