Question: How to inspect a new purchase

I am about to start looking for a dart. I need to know what to look for structurally as far as the unibody is concerned. I can check for rust but I dont know how to tell if the car is a good straight platform to work on or not. Also I dont know any of the specific things to look for that can be a real problem down the road.

One thing I was wondering is if there is any way to measure the cars subframes in relation to each other and if so what are the tolerances. I was thinking that measuring from corner to corner of each subframe in an x pattern and then comparing the two measurments, but I dont know how much the tolerances are to make a car acceptable. I also know that how easy to open and close the doors, trunk lid, body panel gap widths and thats about it. I am planning on buying a car, instantly installing subframes and a six point roll bar to make a stiff chassis for a car built to handle well in the corners. Not too worried about K frames(I already have what I want to use), and I pretty much know all the mechanical ends to check other than the actual body itself.\

Thanks