Early Cannondales had a problem with frame life. I don't know when that was corrected, but I believe that from the early 90's on they were pretty reliable. I don't know about the 86, though.
The problem with old aluminum frames is that when they get enough stress cycles on them, they crack. It would be impossible knowing the expected lifespan of the Cannondale in your photo without knowing more about how much it was ridden before you bought it.
However, in my opinion, this is *way* too expensive for an 86 Cannondale. For $550 you could get a much newer bike, or get a vintage bike with a lot more character than this. IMHO this is a $100 bike, at most, *after* I got a Cannondale expert to tell me that the 86's were OK.