A crashed car is gonna have a lot of squeeks and rattles. Once a car has been hit hard enough to declare it totaled by the insurance company (which is all you'll find) It will ruin the rigidity of the frame and the whole structure won't be sound even if you were to add a roll cage! Think of it like this take a pepr clip and bend it in half, if you keep bending it it gets easier and easier and eventually breaks in half! Aluminum bends and you cannot bend it back without breaking! Now if you replaced the bent parts completely, then the welds are the judge of character.
Also, buying individual parts for those cars is a nightmare..You pay out your ***! In the long run, your better off buying a complete running car and save yourself the headaches involved because it's more than you think to repair a salvaged car!