You left out the most important factor (seriously)...
LOCATION.
Back when I lived up in the northeast, that was, IF frame measured straight, about $1000-1500 car to me.
Now, being located down south, that body becomes a $300-500 shell with value added for clean interior, maybe $200-300.
My inspecting advice:
1) Look at the c channel part of the frame each side below the door. This is one of the weakest parts of the frame, in between the jack point behind front tire and before jack point in front of rear tire. IF that is straight move to #2
2) this is a cutlass so it should have at least some of the braces. You can measure distance (or look for signs of stress/bend/fatigue) from the hole on the front cross member to the hole on front frame horn. Left bolt to left horn hole, right bolt to right hole are equal both sides. Then measure distance between the two frame hole centerpoints.
Bring a piece of board, shims, and level. Park car on level ground, place board running left to right under both front frame horns. Just use your shims to get the board level.measure board distance to bottom of frame horn tip (at bumper shock mount) for each side. This should be equal.
For good measure you can take diagonal measures of core support to body mount, but, the earlier tests i mentioned are more than sufficient to raise red flags that mean walk away.