few questions to sec purchase

Hi, I play with the idea of getting an old sec and have a few questions about it. 1. what known faults does the car have, what must you pay attention to ? what must you lay down about for a sec which should still hold the next 10 years ? 2. what kind of consumption should you expect? 3. is the 420er probably to be converted to gas ? have no idea how and whether this can be done. there are also conversion sets or so ? while it is possible e.g. with my uncle the learned kfz master is something like this n couple of festive evenings to rebuild ? well my uncle hasn’t worked on kfz’s for the last 10 years. what would build inside workshop cost ?