If you absolutely must collect them all, go for it!
I for one definitely understand that line of thinking.
I'm just saying you don't actually need to do that for the game's sake. If you have some underlying subconscious affliction like I do them by all means buy all the ships.
To the OP, have you looked at the Genesis Starliner? I have read that it may have the modularity to allow it to act as an effective troop transport. I don't think it was mentioned here and might be another option to take a look at.