While my experience does not exactly match what you are currently researching/inquiring about, hopefully you will find this of benefit:
I was also originally researching those same options last year for a 3 month trip, but out of JHB, not Cape Town. In the end I purchased a 1996 Land Rover 110 300 Tdi (2 door) which contained most of what we needed for an overland trip, and the rest was very easily sourced. And in the end threw out the original plan once underway and continued right back to the UK.
There was a company there (I thought it was Drive Africa but seems not) that both rented expedition LR's as well as sold ex-rentals so such an option seems to be widely available. However, by our calculations it was going to work out to be much more expensive to rent than to purchase outright (which also gave us far more flexibility in our travels - as we took advantage of). The ex-rentals they had available for purchase were few and far between and all Td5's - which for my experience and preference I would not go with. We also met some young guys in Zambia who were renting from that same company and boasted how they had never dropped below 160 kmh across Tanzania so I would be concerned about how the veh's had been previously treated.
I was also very tempted by a terrific Series III, which seemed to have a much more ... romantic or old-school appeal for such a journey. However I am 189 cm's and though it was great fun to drive and certainly looked the part in the end I couldn't imagine sitting in such tight confines for the long miles ahead. And given what we experienced, I was always very thankful for the long wheel base, long-range fuel tank and on-board water of the 110.
We found quite a few options for purchasing through SA AutoTrader and the various Land Rover owner forums there - as well, many members passed along their contacts so we had a wide network to work with in choosing the right truck. In the end purchasing and transferring ownership/registration took just a matter of days and was surprisingly simple and painless.
The only warning I would raise on buying your own is the need for a Carnet should you be heading into countries where it is required, as you will need to leave a 100% deposit with the AA (on the book value of the vehicle, not the purchase price with all the kit included) - this is returned to you once back in SA. See Carnet de Passages en Douane - The AA of South Africa
for further info.
And if you're looking (early) for an outstanding truck, I'm currently driving mine back to SA down the East Coast (went north up the West) and will have it for sale there in September ...
Any other info I can provide just ask.