Do you think it might be possible that the previous owner might have carried loads significantly in excess of the 10Kg allowed, perhaps over very rough roads? It is your bike and your money so of course welding is an option and will usually be cheaper then new parts. I suggest you think about how you want to use your bike. If resale value is the priority, then new parts are the way to go - or remove the carrier altogether. If carrying light loads suits your needs, then a weld will get you out of trouble, and you can remove the repaired parts before resale if you think they are unsightly. If you want to carry heavier loads, then chat with the guys doing your welding. Most of them know their way around force dispersal and are able to identify where reinforcement would prove useful. It will be ugly but functional.