I've owned at least one model from most series of 911's beginning with 1972,
but missed the "964" series (1990-1994).

Alan's advice is pretty much on target:  The older cars are "older cars" and
if you're into classics and will accommodate their eccentricities and aging
replacement needs, so be it.  However, if you want the benefit of the most
modern engineering, then you buy the latest model you can afford.

If you're in a warm climate, I'd certainly opt for one of the water-cooled
996 or 997 models.  The A/C really works and the engine temperatures won't
give you a headache.

Best older models:

'87-89 Carreras (not incl the '89 C4)  These are the first Porsches with
Bosch's Motronic ignition/fuel injection and these years also got a new
transmission with a hydraulic assisted clutch.  These are also the last
911's that are reasonably easy to work on without all kinds of electronic
diagnostic equipment.

'96-98...These (993 series) are the cream of the air-cooled cars and seem to
be holding their value.  Note that the '98 cars are all Carrera-S models,
built with the wide body...similar to the Turbo.

'01-04...By '01, Porsche had corrected the nuisances of the early ('99-'00)
996 water cooled cars.  The '02's on have an extra 25 hp, compliments of the
engine being 3.6 L as opposed to 3.4 L.

'05-07...These 997 models represent some significant engineering advances.
The "Carrera-S" models have larger, 3.8 L engines with 355 HP.  I just got
one (06) and it's simply gratifying to drive.  Improved handling/steering,
improved interior ergonomics, and lots of torque and HP.  They're expensive
right now, but in 2 years...if that's your time frame, the '05's and '06's
should be down from the stratosphere.

If you live and drive in snow, consider a C4 (all wheel drive) model.

And...as Alan noted:  Don't buy any used Porsche without a comprehensive
pre-purchase inspection by a reputable shop....preferable NOT the one who
regularly services it.  You don't want any nasty (read that expensive) post
purchase problems as poorly maintained Porsche can become a "money-pit".

Join PCA (Porsche Club of America)...you'll have a local chapter to learn
from and get the national magazine and access to the website "market" where
many of the better maintained cars are posted for sale.

Good luck and regards from Texas!