We'll say that the Soviet Battle Group comes in with the express aim to kick the South Africans (1st) out of Angola and (2nd) to ensure an independent government in South West Africa Territory.
That then is a problem. For both sides. The Soviets will have the task of garrisoning a massive, almost roadless hinterland that also has a bunch of choke points along the way SWA's capital, Windhoek, is one of these - its in the middle of the Auas Mountains, and if the South Africans are determined, they'd drop a brigade in Windhoek and hold the lines north from South Africa. Defenders have big advantages, and if the South Africans are prepared well, two brigades in Windhoek could stop a Soviet Division. The South Africans by 1987 had the Ingwe anti-tank missile, and would by early 1989 probably also have the Rooivalk attack helicopter, if they are rushing to get it to the line. Olifants aren't gonna stop T-80s, though Ratels on BTRs would be a fair fight and South African Rooikat or Eland armored cars will tear up BTRs like beer cans. The G-5 or G-6 artillery guns are bad news for the Soviets, too. The South Africans would probably not want to hit hard with nukes over Namibia, especially if they'll kill thousands of Russians if they succeed. That's just asking to have Pretoria or Johannesburg disappear in a mushroom cloud. But if the Russians went to run on Pretoria, the Apartheid state would probably say fuck it and use their A-bombs.
The Soviets will have a supply problem, even if they can land ships at Benguela, it's still a very long road to the front. Landing at Luanda makes that even worse. If they want to overwhelm the South Africans, they'd need to take Walvis Bay and Swakopmund to supplies going. The South Africans will know that, too, so you can expect a lot of South African firepower at Walvis Bay. The South African Navy's fast attack boats would be a problem for the Soviets here, too - they can't operate far from shore, but if they Soviet freighters get cut up by South African fast-attacks, they are just as screwed.
Also, one probably couldn't escape the fact that both South Africa and Israel (especially South Africa in the 1970s and 80s and Israel in 50s and 60s) have these very real feelings of isolation and defensiveness that are far from in their heads.
That's more true than I think you realize, MacCaulay. That was the reason they fought SWAPO for so long, they felt that giving up there would give the ANC a convenient place to launch attacks from. A National Geographic article from 1982 quoted a SADF soldier saying exactly that - "We're fighting for OUR homeland." South Africa supported Rhodesia for a while on that same premise, until Vorster got a comment that black Africa would tolerate apartheid if they could bring down Rhodesia. (And I imagine had Vorster not died, he'd have been very, very pissed at Mugabe for lying through his teeth on that.) It's also why they supported RENAMO for so long, too. That isolation was rather real, too - EVERYONE hated them by 1987, one of the very few things Washington and Moscow could agree on.
But for now, let's suppose that Pretoria decides to try a conventional solution in the hope that one can be reached. You're the head of the South African Air Force or Army: what can you offer?
Get out of Angola, for starters. You can't stop a Red Army tank brigade with Olifants, and there is way too much open ground in Angola, as well as local opposition. T-72s would be too much for the Olifants, let alone the T-80s the Red Army was using by 1987. Best hope is pull back into Namibia, probably also ditching Ovamboland and the border areas - thus removing 2/3s of Namibia's black population from the equation as well, because they are concentrated in the North. Focus on keeping Windhoek, Walvis Bay and the supply lines. Fortified in the Auas Mountains around Windhoek, the Soviets will be facing a tough, determined foe on ground of his choosing, which he will almost certainly have garrisoned. Pretoria would demand that the UNITA attack anything Russian there, and probably provide them with plenty of firepower to do so. A prepared defender can stop three times as many units as he has, which means a South African brigade there can stop a Red Army Division, assuming the South Africans find a way to stop T-80s. Ratels with Ingwes, and anti-tank missiles on ground-attack aircraft or even rigged to their Alouette III helicopters, might be able to do that.
If the South Africans have some time to deal with this, I can see them clandestinely buying a bunch of Merkava Mk II tanks from the Israelis and getting them run out to South Africa. That would be tough to do and expensive, but it would probably even the odds, though I'm not sure a 105mm shot from a Merkava would stop a T-80. I'm also seeing those South African Merkavas getting the laser rangefinder and other gear from the Rooikat, too.
As for the Air Force, that depends on what the Soviets bring with them. If they have MiG-29s, the South Africans have a major, major problem - the Cheetah was even with the Fulcrum on avionics technology but way behind it in most other ways, and its the best the South Africans have got. Their Canberra bombers and Buccaneer attack aircraft could be a wild card, though Soviet SAMs are very good and this would probably result in substantial losses. Against MiG-23s, the odds are more even, as the Flogger is a shitty dogfighter and the Mirage F1 is a rather good one, though the Flogger has radar-guided missiles to compensate for that.