Well, for a tech model lod i'd say put enough detail that adding more detail wouldn't be a siginificant diference, without going insane on the polies. My tech model helios is areound 3000 triangles.
THe ingame model should obviously be much lower, for example, my ingame helios is around 300 tri's
So for the tech room i's say fiddle with between 6 and 8 sides and model in all the beveling on them, for the ingame model, 3 or 4 sides should do it with not all the beveling modeled.
In the tech model, you can use about 3 sides for the "second cylender"
Also, be sure to give it 4 thruster points!
And i think you can cut a bit back on the number of faces in the frontal sphere.