Interestingly, I was thinking of doing something myself in terms of Congo mercenaries of the 1960's in part because one of my uncles was in the Belgian Para Commando regiment in the late 50's and saw service in the Congo. His unit later participated in Operation Dragon Rouge, 1964 where 320 members of the regiment jumped on Stanleyville to recapture it and rescue hostages.
Another reason is my love of the film "Dark of the Sun," which I have mentioned before.
There are problems however with doing a Congo mercenary or Belgian paratrooper model for the Congo crisis and it's that none of the essentials are readily available.
Belgian paratroops (and a lot of Congo mercenaries) wore cammo smocks based heavily on the British Denison smock of WWII, but with at least three different camouflage patterns; Belgian brushstroke, peau de saucisse or ball and moon, and Belgian Jigsaw (still used today). The first of these, the Belgian brush stroke bears a certain resemblance to the brush stroke patterns on British Denison smocks, and so lacking anything else a British WWII Denison smock, readily available in 1/6th scale is about as close as one came come to a Belgian smock.
Another popular cammo pattern that you see in a lot of photos is the French Lizard pattern cammo worn by French paras from the early mid-fifties to the late 70's, including the parachute assault on Kolwezi in 1978. Good luck finding this in 1/6th scale. I did see one figure, representing Alain Delon as he appeared in "The Lost Command," (Les Centurions) that's it. French lizard pattern cammo is something seriously missing in 1/6 scale, given how many countries used it!
If doing a Belgian paratrooper or Commando, the trousers olive of stone color and the boots were the British short type ankle boots with puttees. Webbing also appears to have been British style. 1st & 3rd battalions of the Para-Commando Regt. wore British style maroon berets, while the 2nd Bn wore commando green berets. The distinction lies in the origins of each battalion, but is a minor one as all battalions received the same training.
Weapons are more problematic. The most common rifle was the metric pattern FN FAL rifle and I have yet to see any of these in 1/6th scale. I was able a while ago to get a British inch pattern FAL in 1/6th, and it's very nice, but there are noticeable difference between the two types. This being said, it might be possible to kitbash and convert the British pattern FAL to an Belgian one....
https://www.pinterest.ca/pin/517069600947260255/
The FAL was used both by European troops deployed to the area, UN forces, and Moise Tshombe's Katanga rebels, who look as though they were supplied by the Belgians. I'm pretty sure they were, actually. (Just to further confuse things, Tshombe later became Prime Minister of the Congo until overthrown in a coup by Colonel Joseph Mobutu, who later renamed himself Mobutu Sese Seko.
For sub machineguns alas, I have seen no 1/6th scale Belgian Vigneron SMG's ever. The MAT-49 is a possible weapon for mercenaries or other factions, though I have yet to see a photo of one being used in the Congo Crisis. Other possibilities would be the British Sterling, and definitely the Sten, which I have seen photos of for this period. See below.
https://www.pinterest.ca/pin/517069600949977008/
The mercenary in this photo appears to be wearing a "ball and moon" pattern Belgian smock, British style web belt, though I'm not so sure about the suspenders, a Sten gun and wearing a pattern of bush had frequently work by Tshombe's mercenary troops.
https://www.pinterest.ca/pin/517069600947260436/
The mercenary in the above image is wearing a Belgian "Jigsaw" pattern cammo smock.
https://en.wikipedia.org/wiki/Operation_Dragon_Rouge#/media/File:Dragonrouge1.jpg
Members of the Para-Commando Regt, carrying their FN FAL's, but can't make out the pattern of their smocks.
https://www.pinterest.ca/pin/373869206544870171/
Mike Hoare, the famous British mercenary who served the Leopoldville regime.
https://www.pinterest.ca/pin/644155552934525628/
A number of mercenaries had served in the Wehrmacht during WWII and wore their German medals, which fact was caught on by the world press.
So as you can see from the photos, if you are trying to do a Belgian paratrooper or commando, you might find it quite challenging, but as far as mercenaries go, you have a broader canvas with which to paint.