An offer (by the dealer),
An acceptance (you agreeing to the deal)
And consideration (payment of at least a portion of the sum owed).
So you paying the deposit effectively completes the last two.
The fact that it is conditional on the outcome of the inspection offers little protection unless you specify what is 'satisfactory' and satisfactory to who?
It's a bit like the old "lots of interest in this car, but you can secure it with a deposit" line from salesmen. To which I will normally respond "gee, that's good for you, hopefully it will sell quickly to one of them".
Unless the vehicle is a one of a kind, you hold the cards in this circumstance. Not completely happy -> walk away. But don't part with any money to the dealer until you are happy to do the deal. As mentioned above, pay for the inspection directly to the agency doing it (AA, VTNZ, etc).