Problems with first truck purchase

Hi all, greetings from southern Australia (Otway Ranges, Victoria). Recently bought my first truck: a 1979 Mercedes Benz 1217 25 foot tray truck. Was aiming to use it to cart cattle and hay. Thought I got t a great deal til a mechanic found several cracks in the chassis. Laws are pretty strick in Oz regarding repairing such problems. Got a quote today for $15k to replace both rails plus $2k for engineer to sign off. This is so I can get a roadworthy certificate for registration. Any Aussie truckies out there with any ideas?