I want to know what is the official survey datum, and projection system for Zimbabwe? I think it is ARC1950, UTM Zone36.
Is it better to convert all the old coordinates to WGS LO29 (LO depending on nearest uneven longitude)?
I am unable to find any official sources, but I believe you are correct about Arc 1950 / UTM Zone 36S (epsg.io page for reference). It's a longshot, but perhaps you could contact someone at the city of Harare, where they are implementing a GIS system, and verify what they are using. It looks like the City of Munich, Germany is assisting them with this.