Dan,
I' ve just (in March this year) been through the same - I started by looking for only 1992 cars or younger, and very quickly found that the model year of the car is really unimportant. I looked at and drove many cars before considering purchase - some newer lower mileage cars were actually much worse in terms of condition that some older, higher mileage cars. The golden rule is get a pre-purchase inspection done by a specialist - or do what I did, buy from a specialist who guarantees the car. I ended up buying a 1990 C4 coupe from 911virgin.com in uxbridge - it was by far the best car I had looked at in three months of travelling around. In the end I went more for a mechanically well sorted car that was perhaps cosmetically not as nice as others I had seen - mainly because cosmetics can be sorted out without re-mortgaging your house, but buy the wrong car, and the mechanics can cost you a fortune. Look for a car that has a really good history of money being spent on it regularly, not just FSH, but evidence of other non-essential work like 4-wheel alignment. If it' s a C4 look for 4 matching make / tread good tyres. Try and speak to the previous owners to get a feel for how enthusiastic they were about the car. Read as much as you can - there are several books, Red Book is good for checking vin numbers, Buyer' s guides etc.
Good Luck - it' s well worth all the effort to find a good one!
Cheers,
Pete.