Have you read the 914 faq?
http://www.pelicanparts.com/MotorCity/siverson/914/914faq.html
It's a good starting place. I don't think there are any bad years. Aesthetically, some people prefer the earlier cars because of the bumpers. The later cars have a little less power because of smog equipment. Your decision should take in to account local smog requirements.
More importantly, you don't want a car with expensive to fix rust in hidden areas (under rocker panels, under battery tray (including rear suspension), behind engine firewall sound proofing, etc).
I'd recommend that you buy the cleanest car that you can afford. If you buy a $3000 car today with the intention of fixing it up, you will certainly spend more than $3000 to make it look like a car you could buy for $6000 today.