You have left out almost every conceivable detail of the circumstances of the purchase other than to say you "bought a Ute Privately" and expect legal advice?
For all we know, you could have tripped over a Ute while walking down the street then decided to throw $6500 at a bystander and drive off in it because someone left the keys in it! Nothing in your post to suggest otherwise is there?
What we will want to know is how the vehicle was advertised for sale? and what claims were made in the add? and do you have a copy of the add? If the seller made no specific claim (provable claim) about the condition of the Ute then you are out of luck.
As for the mechanical side, how can roadworthy cost be that much?? Sounds like that Mechanic is being seriously anal about it? What do you need for a safe vehicle? It doesn't have to be like a NASA F space ship .....Gezuz,... All you need is good brakes and tyres and all lights working, not blowing smoke, that's pretty much it. Find a different mechanic.