That's a very good point. If I was commanding the South African military and faced with what is essentially a Motor Rifle Division spearheaded by Soviet armour, first off I'd wonder what sort of bankrupt regime would put me in charge of anything larger than a steering wheel. Then after that I'd probably pick whatever geographical feature I knew the MRD would have to advance through, and dig in so as to reduce the tanks' mobility as an asset.
Then I could offer battle on as close to my terms as possible.
Probably some sort of mountain range straddling a road, or perhaps a river crossing.
That fact gives the South Africans options. Windhoek, the capital and economic center of the SWA Territory, is surrounded by the Auas Mountains and getting tanks through mountains is a pain in the neck. The Soviets would be forced to move along the main roadways, and their supply lines would be a bastard. They'd lose a bunch of tanks between Luanda and the SWA borders from mechanical attrition (a well-known problem with the T-80, which had engine reliability problems right from the off), but most would make it. The Soviets will have two options here. The South Africans are not stupid enough to garrison the border with Angola against a Red Army MRD - they'd get eaten alive, and giving up much of the northern part of the SWA Territory makes it easier because of fewer guerillas. Windhoek would be the key, as all of the roads lead there. If the main thrust is coming from Angola, they'll have to go across the border and SE around the Etosha salt flats (Don't even think about running any tank or armored vehicle over a salt flat. Really.), taking the towns of Tsumeb, Grootfontein and Otavi, before going south. The B1 road is the only way to move a major mechanized force southbound, and it has multiple spots where a South African commander could cause problems.
Key spot #1 is around the town of Otjiwarongo. The town has an air strip and great transport infrastructure, but has mountains on three sides, and a dedicated South African commander could cause a major problem there. The one problem for them is that there is two good roads going out, though blocking both is possible, but it would be a problem because moving the force to support either blocking force would be a pain. Either way, one has to go uphill.
The opportunities for blocking forces are many, which the South Africans would know - and the absolute worst one is at Windhoek. Windhoek to Rehoboth is about 90 kilometres (55 miles), but its going straight up and over the Auas Mountains and would be a nightmare for the Russians. If the South Africans can hold Windhoek, South African airstrikes, artillery fire and commando raids would make life a bitch for the Russians, and it would be easy to do in all cases. There is few roads worthy of the name in the hinterlands, and there is no direct line between Swapokmund or Walvis Bay and Windhoek. The line would end up stalemated between Windhoek and Okahandja. The Russians could try swinging around to Gobabis, but the only rail lines goes to Windhoek and the roads around there suck.
If the Russians were willing to force an amphibious landing, they would be advised to hit Swakopmund and Walvis Bay, which would put them several hundred miles closer to their objective. An MRD would need the port at Walvis Bay to keep supplied in any case, because trying to move enough fuel from the ports in Angola is going to be a mighty challenge, even before UNITA stirs up more logistical trouble, which they undoubtedly would.
South Africa is gonna have a logistics problem, too. From Windhoek to Upington is nearly seven hundred miles. Keeping the road and rail line to Windhoek from the RSA open is going to be very important, especially if the Russians take Walvis Bay. The only other major port in Namibia is Luderitz, but getting supplies to Windhoek from there is worse than getting them from Upington. South African Railways would have its hands full, though SA's rail net to Upington gives options - move supplies southwest to it from Pretoria and Johannesburg, due west from Bloemfontein, Northwest from East London and Port Elizabeth or Northeast from Cape Town. Beyond Upington, however, you have one major road and one rail line, and only a small number of rudimentary airfields.