We don't know exactly what kind of shape it will be in. There are a few similar objects that have been returned, however, that we can get at least an idea of what it might look like. Let's look at the comparisons:
- Surveyor 3- Landed on the Moon, was visited (Ironically) by the crew of Apollo 12. A report was done on the damage to this craft. 2 years on moon.
- LDEF- Basically this was a satellite placed in to orbit by the space shuttle, and recovered years later, to see what would happen. 6 years in LEO
- EURECA- Same thing as LDEF, but from ESA, not NASA. 1 year LEO
- GENESIS- Solar sample return, crashed on Earth after 3.5 years.
- Stardust- Comet Dust- small capsule returned to Earth. Deep space, 7 years.
- Hayabusa- Asteroid sample return, 7 years Deep Space.
- Many of the missions to deep space have included a dust counter. I'm going to look at the Venetia Student Dust Counter, on New Horizons.
A few other things. The dust environment in interplanetary space is quite a bit less than that around Earth. I would expect there to be some small impacts, but nothing compared to the LDEF mission, for instance. The dust levels seen in space have been measured, look at the SDC from New Horizons.
The dust levels are quite low, so we can assume that there will have been some, but not a significant number of small impacts.
The radiation is of some note. There is a good article about the effects of paint in space. Specifically, it states:
Ultraviolet radiation is particularly damaging to white paints used on the external surfaces of satellites. UV radiation causes significant increase in the solar absorptance of white paints, while it decreases the absorptance of black paints due to bleaching effects.
Bottom line, the black paint will become lighter, and the white paint darker.
Lastly, there is some indication that the paint, might bubble, which would lead to some chipping in the paint over time. This is based on the bubbles seen on Apollo 17.