Regarding the "PPI", here are some recommendations listed by "RALPHIE" a while back:
"Rich & Denis - "The GT Guy," just south of Detroit in South Rockford, Michigan
Shelby Smith - "Elite Motors," Jonesboro, Arkansas
Rich Roback - "Invisible Touch Enterprises," Kingman, Arizona
Bill Knobloch aka "Shadowman" - "Discovery Automotive," Placerville, California
Kendall Pond aka "nota4re" - "Cooltech, LLC," Newhall, California
Plus the Hennessey Dealerships in Sealy, Texas and Lake Forest, California, and Heffner Performance in Sarasota, Florida. Perhaps Kip Ewing in Royal Oak, Michigan."
You won't have to concern yourself with "what to look for" (in the sense I think you mean the question) if you have an expert check out the car you want to buy.
I would add: Don't accept as 'gospel' any info "Car Fax" (or whomever) gives you. Many GTs have undergone various degrees of repair that just don't show up on reports anywhere. That's the main reason a PPI has been suggested.