Hifi W123

Servus! I have a rather rare question (seen at least a few threads about Car-Hifi in connection with this car) And I would like to add an entry-level sound system to my W123 (Limousine). Price limit would be at about 300-400€, if there is more money that is much better, please suggest that too. I am an absolute newcomer in this field and wanted to get advice here first. If there is a sub in the price it would be all first cream, but I need a find another radio (the built-in radio from anno is not to be used at the time I think ) Also I would have to arrange this so that the components are installed invisibly (H-approval), I thought about radio in the glove compartment etc; to be necessary I just have to install the original radio before every TÜV visit again. I look forward to your answers, LG Carlo