What regulations apply to a VFR flight from South Africa to Namibia?
For SA to Namibia:
Factors to consider

Buying all the appropriate charts.
Clouds: if you can't maintain the required separation, you need to switch to IFR, which means the plane needs to be IFR-equipped, and for you to be IFR rated.

Sources: Namibia AIP GEN 3.2 and ENR 1.2

En route airway paths
All airways from SA to Namibia are Class A, so VFR is not allowed along ATS routes.
Source: Namibia AIP ENR 3

Permission
You need overflight permission from Namibia, else there's an up to N\$50,000 fine or 5 years imprisonment. It is submitted 24 hours before.
Source: SA AIC D 012/2014

Flight plan – yes
Across international borders flights do require flight plans (the ICAO type) in accordance with ICAO SARPs Annex 2 (Rules of the Air), which both SA and Namibia require.
Sources: SA and Namibia AIPs GEN 1.2

See also:

SA AIP
Namibia AIP


For any other countries that share a border, refer to the respective AIPs.