The main issue is always the number of polys, as the less polys you have, the less curvy the sphere is, and the less accurate will that kind of calculation be relative to the real model shape.
I don't think only 200 polys will cut it, especially given the size that you want which is pretty big.
Since it's a single model, I would instead advise to make it at least 1000 polys, as the engine can handle some 1k models in view just fine.
Spheres have a bit more science to them than what they look at first, especially when your next worry is UV mapping: some ways of building spheres are more UV mapping friendly than others.
The ultimate one is making a sphere out of a tessellated cube (this is what games like No Man's Sky do to generate planets).
For whoever's interested: https://medium.com/game-dev-daily/four- ... 956b825db4