I absolutely would not buy a car that was "not certifiable" for any reason. It's probably not safe at any speed. If you're looking at a car that "will" or "should" certify, Then I would try to make arrangements with a chassis shop near the car to have it checked out before you buy it. If the owner is so sure of the car then they shouldn't have a problem with it.
I bought my altered off of here. It had just recently expired both NHRA and IHRA so I was pretty confident with the car. I had a new top put on the cage simply because I was too tall for the car. It's was then certified with no problem.