I have issues with the classic grey goo, in that I think the thermodynamics holds up.
This is basically life. This is what life does.
This is going to take tremendous energies and similar amounts if waste heat and a lot of what comes out will not be in the ground states.
I kinda figure, "practical" von Neumann machines might be indistinguishable from anything living. Sure, you could imagine we're building them from metal or something, but metal is so much rarer than carbon. And organic molecules are the sh*t in potential self-organizing capabilities and stability and what not. I guess there is a decent possibility that there is some other chemical composition that works better in vacuum, but it could be your probablem would be building a self-replicating machine that needs less common materials. (And even carbon-based life still needs rarer materials - but also comparitively less. Once you want something like mitochondria, you need phosphor)
Silicium-based life has been hypothesized, but at least my cursory "internet research" suggests that the devil is in the detail, and many more complex molecules build from it wouldn't be as stable as their carbon-based equivalent.
The worst case is that interstellar spaceship looks like a star system and is more like a spacetrain, with gravity defining your rails, and you don't take a lot of stops...
For "how common" is life:
Now that we have finally found the first star systems, we found a lot that don't like ours. Due to the technical limitations we're working with, we are more likely to detect more massive planets than ligher planets, so it doesn't mean our type of star system is unique. Yet, but it is still interesting that most star systems we found look different from ours, including having a planet type not found in our system. Super-Earths are rocky planets like Earth, e.g. they are not gas giants, but they have far more mass, which makes certain aspects of life harder to accomplish (if not impossible). And we also found Water worlds (or at least one), which sounds cool first, because "Life needs water", but it turns out the water is so deep that the pressure creates a barrier that doesn't let through minerals and elements that are needed for life, so it would probably need to come from asteroids (but it still drops to the bottom eventually and might still become unavailable.)
Our star system has at few general features that make it rarer:
- a G type main sequence star (a common star, but less common than red dwarves, less massive starstheir habitable zone is closer, but their emission seems to vary more, which could ultimately mean that while the average temperature is okay, the sudden changes wipe out forming life.)
- rocky planets in the habitable zone
- some water on a rocky planet in the habitable zone
- Big Gas Giant that helps deflect asteroids from rocky planets in habitable zone