If you buy a 1964 Chevy Impala 4 door for 600 dollars are buying rare vehicle or piece of junk?

For $600, you're almost certainly buying a piece of junk, not a rare vehicle. A 1964 Chevy Impala is a desirable classic car, but a running, roadworthy example in decent condition would sell for many thousands of dollars. $600 suggests a car that needs extensive, likely costly, repairs or is missing major parts. It might even be beyond economical repair. While it *could* be a rare variant in some exceptionally poor condition, it's far more likely to be a project that will end up costing far more than $600 to restore.