Don't see why 17" tyres have to be skinnier than 18s? Usually it's just the sidewall ratio that changes and the width is the same, to retain the same overall diameter.
If you are going to do a lot of work on unsealed outback roads you should perhaps consider some 17" all terrain tyres with LT construction.
This will give you much better protection against sidewall damage, especially beneficial if you don't have a spare.
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