Jeff Stork is an active member of  The Cadillac Maiiling List.  He is a proud resident of Studio City, California and has had a lot of interaction with the automotive and movie industries.  He consistenly amazes fellow list members with his ability to get "Cadillac Style at Wholesale Prices."  He owns a number of magnificent vintage Cadillacs, and has liberally shared his wisdom and criteria for purchase with the list.

A compilation of this wisdom was mailed to the list at my request ("What ARE these Stork Rules that everyone keeps talking about?"), and I reproduce that list here.  It is slightly edited for clarity.

1. It must be a one owner car. If possible the one owner should be elderly and female, and the car should have been stalked in the parking lot of the local church, bingo or senior center. Otherwise, stalk cars from the estate of a rich Beverly Hills widow or a Saudi Arabian oil sheik.  Better to find the car in LA than the Middle East - lower transport costs.
2. It must have low to ultra low miles - low to ultra low varies for each buyer. Low for mere mortals like us is 50-70K. Low for Stork used to be 30-50K. Jeff's latest is under 15K. He likes to keep it challenging for himself.
3. It must have been carefully stored.  In order of desirability (most to least)
• dry, dark garage - never exposed to sunlight
• dry, dim garage - sun rotating and never hitting only one spot (no selective fading)
• garage of any type
• carport.  Highly unlikely that stork would buy a carported car, unless it were a 1971 coupe in Jennifer Blue for which he might make a reckless exception.  Another exception may be if it were purchased in Palm Springs where carports are more the norm.
• street parked (only if in desperate need of a fix - and we never let ourselves get that desperate)
4. The interior must meet two points - both equally important - PRISTINE and ORIGINAL - no negotiating on either of those. It's painful sometimes to let an almost great car with small cosmetic flaws go, but in the end you'll thank yourself and Stork for adhering to this one. As soon as you buy the almost great car, the actually great one comes along. 
5. The "Nevers"
• $5000 rule - never pay more than $5000 for any 70's car
• Never be in a hurry to buy - let the car come to you
  

I can vouch for these rules.  I have learned that it ALWAYS costs more than you expect to rehabilitate a car, and you will kick yourself later for not spending the extra time and money up front to get a better car.