Structure as in subframe connectors to tie the subframe to the rear frame rails and at a minimum the "X" brace that the factory bolted across the floor pans under the driveshaft.

Rust will be a big concern. Check floor and trunk pans; the bottoms of the front fenders and rear quarterpanels, rocker panels, under vinyl tops, paint bubbles, noticeable patches. It might be worth your while to invest in a paint check magnet.

Numbers are good to check too. If it's a neighbor and the car's not on the open market - getting info from the Pontiac Historical Society is a good investment.

A good aftermarket A/C system company is Vintage Air. Replaces the entire stock HVAC system with one that fits under the dash (including A/C)