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So my late uncle was working at Siemens and had a "little" home workshop/lab with resistors, capacitors, ICs, switches, soldering boards, wires and what there is. He learned something regarding radio (german: Nachrichtentechniker I think). Among others there was a self built radio clock (that got quite warm, plugged it out bc of safety concerns)
Now I am getting into microelectronics and roughly know what there is. But I only know that I should not use the leaded soldering tin (bc lead) - is there anything else that is unsafe because of old standards or aging? What should be safe to use?
There's nothing wrong with using leaded solder, you just need to wash your hands after soldering. Many of the components made before RoHS will contain lead too.
Old capacitors may be no good. Electrolytic capacitors go bad with age. You can measure the leakage and ESR to see if they are still good. Any paper capacitors are going to be very leaky by now. If there are any oil filled capacitors from the 1970's or earlier, they may contain PCB oil, which is very toxic.