Is there any reason why you suggest diesel tugs?......petrol will do the job just as well plus on an older vehicle will be less costly to maintain especially in servicing costs. They are also cheaper to buy given the year model.
For 6K you can buy a BA or BF Falcon sedan/wagon both which are excellent tow vehicles with half the mileage of a diesel tug plus the parts are very cheap and readily available.
Good Luck