Congo would have been cut out of the international markets which gave her some of her riches (remember Mobutu was one of the richest men in the world even if Zaire was one of the poorest places on the planet) and would probably have been reliant on Soviet aid just to remain functioning. The Belgians might have gotten involved in Katanga and the Rhodesians and South Africans certainly would have caused trouble. I'd guess a civil war lasting most of the seventies, kicking off against in the 1990s due to the fall of communism. What little money there was would be gone and it would be everyone fighting over the scraps. If the 'DRC' isn't balkanised in the 1970s then it probably will be in the nineties.
However, a communist DRC would have interesting effects on its neighbours. Angola would probably have remained communist due to Guevara's and Kabila's continued support, and there would probably be unrest in Rhodesia and South Africa. I think there's a TL about communism in eighties Zimbabwe, but I'm not sure who wrote it but it was quite good.
Another interesting effect would be on Julius Nyerere's African socialism in Tanzania. I think that Nyerere's project would probably go better due to a greater market for Tanzanian products and greater Soviet interest in Africa. Whether relatively-benign Nyerere would want to be associated with the despots of the Congo is another matter, however.
So yeah, I fear that it's probable that the Congo would do even worse under Guevara and Kabila; balkanised (although that might be a good think) and a slave to the communist bloc. I'd say that your best bet for a relatively stable DRC is to have Lumumba survive and for civilians to rule as best as they can, to at least control some of the excesses of the Heart of Darkness.