If I were going to buy a used airliner for a private jet, I would buy nothing earlier than a 737NG or an A320 series Airbus. With the 737 Maxes A32X NEO’s coming online with lots of airlines, there should be lots of 737NG’s and A320 series airlines retired from airline service.
So you’re saying basically, is look at what low end freight companies are flying and then don’t buy that? :o)
And by the way, Airborne Freight was flying refurb’ed DC9-15s back in the late ‘80s.