I've been eyeing this plot in Ridgeworth, close to Tyger Valley (accessible via Bill Bezuidenhout Ave and Old Oak Rd). It was initially listed for R3m, it is now listed as low as 2.34m, 1010m2, it is situated on the corner of Old Oak and Ridgeworth drive close to both Durban Road and the N1. It is opposite the Old Oak office park and Tuscan Park Villas but on the opposite, residential side.
Google Maps Street View (taken 2013) show people working on the foundation:
But it never got past that phase and listing photos show it unkept:
Other listing description says plans were approved for a 15 bedroom guesthouse:
The main challenge is that this is a residential area and you would be limited in what you can build, guest houses aren't really our thing as it is labour and capital intensive to operate but in built-up areas in good locations vacant land in Cape Town is hard to find.