Welcome to the Worlds Best 911 (air cooled) forum.
The "traditional" advice is to buy the newest one you can afford that is in good to excellent shape (Bruce Anderson is fond of this saying). That said, you first need to decide which range of models "floats your boat". some factors to consider:
1-Coupe vs targa vs cabriolet;
2-Which range of models fits your desires
Early (65-73)
Mid (74-77)
SC/ Carrera (78-89)
964 (90-94)
993 (95-99)
996/997
I would suggest you look at Wade Guthries FAQ (
http://www.pelicanparts.com/911/technical_specs/911_guthrie_faq.htm) and read Peter Zimmerman's Book " The Used 911 Story" .
Of course, look here and ask questions as well.
You should attempt to drive the different models and see them at different states of repair.
For your budget, it is often suggested that you may be better served with a most excellent SC instead of a ratty Carrera.
Bear in mind that all of these cars typically need work and you should budget $2,000 for your first year of ownership (plus the cost of any upgrades you may desire) and $1,000 to 2,000 annually for ongoing maintenance. Someyear, your maintenance cost may only be $500 and other years it can be very high (several thousand).
As others already said, a Pre Puchase Inspection by a qualified Porsche Shop is considered to be a necessity. For the older cars (air cooled), stay away from the dealers and find a shop with a older fellow who loves the cars.
Best of luck.