buying a drag car, what to look for?

If either of them have a cage, check to see how it's mounted. There should be a steel plate welded to the ends of the tubes, that mounts to the floor. Often times, there will also be a plate on the underside for a bit more strength, but I think that's usually with bolt-in cages. Take a look at the torque boxes and see if there's any evidence of cracking, or pulling away from the body. Make sure the car appears to sit level when on level ground, and that the body panels (roof in particular) aren't extremely wavy, as that would be a good indication that the car's been flexing too much. If the car has 4 point harnesses, are they mounted to a cross bar behind the seats, or mounted to the floor? Floor mounted will compress your spine and cause severe injury up to, and including paralysis, if a severe enough accident were to occur. If the battery is hatch/trunk mounted, is it in an NHRA approved box? It has to be sealed from the passenger compartment with a battery cutoff located outside the car, and a vent tube running outside the passenger compartment from the box. Other than that stuff, which is all I can think of off the top of my head, it should be all the normal stuff to look out for.