You have three questions here.
1. How high can a geostationary Sweden be visible from South Africa?
Answer: 68,100 km
This is a question that humanity has pondered since we first climbed down from the trees.
The other answers are correct that if two points are more than 90 degrees apart on the globe, they cannot see each other at any height (unless the earth has a big groove in it or atmospheric refraction does something funny, but we don't have to consider that in this answer).
The correct way to calculate angular distances between two points on the globe is to use the haversine formula. This takes both latitude and longitude into account. I'm feeling a little lazy today, so I will just take the distance measured on Google Maps, divide by 40,075 kilometers and multiply by 360 degrees. This is equivalent, and I've checked that the math lines up.
If Sweden goes straight up from the point of view of its center point, the important location of Sweden is Flataklocken at 62°23′15″N 16°19′32″E.
I found a point near South Africa's Zimbabwe that is 9,472 km away from Flataklocken, which comes out to 85.09 degrees.
You're in luck! You can see Sweden from South Africa. It will be low in the sky, within 5 degrees of the horizon. It will be brighter than the full moon since it's a lot closer.
If you intend to actually do this, I'd recommend buying a hot air balloon to take South Africans up to view.
a = 85.09 degrees (this is the angular distance between South Africa and central Sweden)
r = 6371 km (this is the radius of the earth)
find h, the height of Sweden
cos(a) = r / (r + h)
h = r (-1 + 1 / cos(a)) = 68,100 kilometers.
This is over 10 times earth's radius. It's far outside earth's atmosphere, but low enough it's not going to crash into the moon.
2. If Sweden were floating in a geostationary position in the upper atmosphere, how far would it be visible?
Answer: 2400 km
Earth's atmosphere only goes up 480 km, so if you want Sweden to stay in a geostationary position in the atmosphere, it won't be visible from South Africa.
If you want the people of Sweden to have enough air to breathe, it will have to be even lower. I'm not going to run the numbers for that, because that's not the question you asked.
The equation is the same, except in this case, we know h and r, and we're trying to find a.
r = 6371 km (this is still the radius of the earth)
h = 480 km (Sweden floating in the edge of the atmosphere)
cos(a) = r / (r + h)
a = arccos(r / (r + h)) = 21.57 degrees.
This comes out to a distance of 2400 kilometers.
This isn't far enough to see from the southern hemisphere, but you would still be able to see southern Sweden from parts of Algeria and Tunisia. As far as I'm concerned, that's good news!
3. Can a Sweden orbiting in the atmosphere be visible from South Africa?
If you can make Sweden orbit at this low elevation, it can be visible from every point in the world. But you can't make Sweden orbit in the atmosphere.
Orbit means something is spinning around a world without rockets or thrusters because it is going so fast that gravity can't pull it down. You can calculate the orbital speed for any height in a vacuum. Unfortunately, orbit is not possible within the atmosphere because the air resistance will eventually slow Sweden down so that it will crash into the earth.
Don't despair. You can still make Sweden fly. You'll need to buy big jets or something else that uses energy to move it around. I don't know what your budget is, but I think it's still worthwhile. Think of the spectators in South Africa.