Namibia is a fabulous country, not the cheapest but safe and beautiful. It's currency is linked to the south African Rand and is interchangeable, you can spend Rand in Namibia without changing and vice versa, handy when the main tourist base is likely to South African.
Namibia is a relatively large unspoilt county, it has stunning scenery and has excellent game parks, Etosha being one of the very best.
Water is the main issue in Nambia, you would need to ensure you had a bore hole on the proposed farm/game reserve otherwise you would be unlikely to be granted a license to operate. Access to and from SA is easy, the road infrastructure is good, saying that property isn't cheap.
As you say SA can be volatile, and with the political climate being as it is is likely to get worse.
Kenya and Tanzania also offer the potential for good reserves but these are relatively well covered by them now so competition would likely strong.
Zambia has potential for investment, the Northern Province is looking to develop and has few attractions as of now, a game reserve would be ideal, it would attract clients further up north and you should get a reasonable deal on land prices, Zambia is relatively safe. If you are serious about the possibility contact Zambia Ministry of Tourism and ask about their Northern province development plans. The current tourist route are based in the south and east of the country (south Luwanga, Livingstone and Kafue) although tourists are attracted to go up as far as Shiwa Ng'andu (Africa House), however they seldom go as far as Kasama, which they pass the access road to on their way to Shiwa (turning at Mpika), a game reserve Kasama way would be ideal, fit in with local planning and extend the tourist route in the north.
Zambia also has a growing tourism market they have capture much of the tourism that Zimbabwe have lost, that and a thriving economy, Zambia is fast becoming a commercial centre of trade in Southern Africa, and has many expats living there, who always seek new attractions to visit.