Cuban muscle, Soviet logistic
History of Cuban intervention in Angola is relatively well documented, as is the Angolan Civil War. Without going into too much details, it evolved into usual proxy war in the Cold War era, where US and USSR had their favorites on the ground, but wanted to limit their involvement and potential losses, as the region was not deemed strategically that important (it should be noted that Soviets in the end did send some "advisors" but nothing on the scale of Cuba). Therefore, this did not evolve into something like Korea, Vietnam or Afghanistan. However, South Africa consider it as something of primary concern, and Cubans had their own reasons, mostly prestige and bluster. Cuba under Castro viewed itself as a leader of Third World and former colonies, and many Cubans having been descendants of African Blacks do have ties (genetic, historical, spiritual) with Africa.
Nevertheless, question is mostly confined to logistics, so let's address that. In general, Soviet-Cuban economic relations and Soviet aid to Cuba was vital for this country, in so much that dissolution of USSR in 1991 hit them very hard. Soviet military aid was also substantial, therefore Cuban armed force were rather large for the country of this size. This was of course consequence of proximity between US and Cuba, and Soviet willingness to turn Cuba into some kind of frontier outpost and fortress in Western hemisphere. As for quality of Cuban armed forces, in general it could be said that it followed Soviet model. Compared to Western militaries, quality was lower mostly because general lower level of education of officer corpse and additional requirement of political loyalty beside professionalism. Conscripts of course reflected overall structure of the society, but troops intervening in Angola were mostly volunteers i.e. some kind of professional soldiers. It is generally taken as granted that Cuban soldiers and officers were bellow in quality compared to their South African opponents. Nevertheless, Cubans in general were better then African militias involved in conflict, and in the end achieved some kind of victory, mostly because of sheer weight of their effort and gradual weakening of SAR due to international sanctions caused by Apartheid.
Military equipment deployed in Angola by Cubans was sizable part of their armed forces, although sheer number of tanks and other AFV was probably inflated in Western sources. At the peek of the intervention Cuba had around 50 000 personnel, although not all of them were combat troops. Quality of the equipment followed usual Soviet pattern toward their client states. Some equipment was reserved only for Soviet forces. Warsaw Pact countries would get mass produced relatively modern equipment with some features disabled (mostly avionics, fire control system and other sensitive electronics). Third World countries would usually get "monkey models" i.e. again relatively modern equipment but usually with electronics and other sensitive stuff from previous generations (radars, ammunition, protection kits for tanks etc ..) . They were not considered reliable and USSR didn't want something critical to get into Western hands. Cuba was some kind of exception in this regard, for example while Angolan Air Force in the beginning was equipped mostly with Mig-21 and Su-22, Cubans fielded relatively new Mig-23 and supposedly more advanced ML versions. Tanks like T-55 and T-62 were no more state of the art (especially T-55) but were still useful against SAR Centurion(Olifant) tanks and of course even more against poorly armed UNITA infantry. Artillery like BM-21 Grad significantly outranged what South Africans could deploy (except perhaps G-5 howitzers). Overall, it could be said that Cubans were roughly on the level of usual Warsaw Pact country of the period.
Was this a great economic burden for Cuba and Soviet Union ? I would argue that it was not. From the Soviet perspective, they were already producing great quantities of equipment to stay relevant in the Cold War. At the moment we are talking about they were gradually introducing Mig-29 and Su-27 to replace Mig-21 and some Mig-23 in their service. T-55 and T-62 were gradually being replaced with T-72, and T-64 and T-80 to some extent. They had huge stocks of existing equipment and giving it (in return for some favor) to allies was much better then letting it rust. Since they were already giving aid (but also receiving preferential status for their trade) to various socialist countries, it was business as usual for them. Unlike Afghanistan, Soviet troop loses in Angola were relatively tiny. Therefore, it was mostly manageable for them. For the Cubans, situation was somewhat different. They were tasked with first and foremost bringing troops and equipment to Angola, and latter supply them. This was mostly done by ships of Cuban merchant marine which was strained at some point, but overall with the help of Soviet Union and their ships and aircraft problem was solved more or less satisfactory. They were able to supply peek effort of 50 thousand troops. It should be noted that since there was no open war declared between Cuba, USSR, SAR, Angola, US or any other country, there were no attempts of sea blockade or any other large naval action that could disrupt these seaborne transports. Morale of Cuban forces in Angola was regarded as high, and overall trough skillful propaganda and depiction of war in Angola as a war of liberation, Cubans had enough volunteers for service, both in military and among civilians (doctors, engineers, technicians) sent to Angola to help. Casualties are still debatable topic, they are estimated at 15 000 KIA, MIA, WIA and POW. Overall, it seems that casualties did not deter Cubans from participating in the war. Cuba already had relatively high expenses to maintain large standing army (for a country of 9-10 million people in that period) . It was reported that war actually helped with some of these, as USSR and Angola actually participated into cost of upkeep. Again, for Cubans economy turned for the worse only after the war (after 1991) when Soviet economic help suddenly ceased.