Extra thrusters and engines (beyond the minimum of one each needed to launch) will reduce your spaceship's travel time. Generally you can build all available components at the same time in different cities, so there's no real reason not to have a full set. Occasionally, if you have very weak production cities, it isn't worth building the second engine (i.e. if you have completed all necessary research, and one engine will be finished a few turns before the other).
Casings are not really optional, as they affect your chance of success. A ship with only 1 casing only has a 20% chance of reaching its destination. 80% of the time it will fail after launch, and you'll have to start building from scratch. Each additional casing increases the chance of success by 20%, up to the 100% success rate with 5 casings. Again, you can probably build the casings in parallel, so it doesn't take significantly longer to build 5 than to build 1.
Along a similar vein, after rocketry, what is the optimal tech path to follow to get the spaceship built asap?
In general research, not production, is the limiting factor in spaceship construction. Rocketry is therefore not the most important tech, despite it allowing the Apollo Program. Superconductors is generally the starting point, since it permits labs, speeding up the research. As a very rough order I'd go:
1)Superconductors
2)Rocketry
3)Plastics
4)Satellites
5)Computers
6)Fibre Optics
7)Fusion
8)Composites
9)Genetics
10)Ecology
If you're using the religious wonders a lot you could swap Computers for The Laser.
One last point - avoid the space elevator as it is worse than useless unless you have a tiny number of cities. The reason is that it requires a non-essential tech (robotics), which will take longer to research than the elevator will save on production time. Remember that its research that is the limiting factor, so the elevator is only saving turns on the launch date on the very last component (which would be the cheap Life support system in the suggested path above). If you can steal robotics from an AI, rather than researching it, you might be able to save a turn or so with the elevator.