A couple notes. If it's a shortbed you want, 1/2 ton is your only option. There were no factory shortbeds other than 1/2 ton.
You are in a great area to find a solid truck. Patience is the key. Keep watching, the nice trucks at low prices probably sell quick, so you need to be checking craigslist daily.
www.searchtempest.com is a great way to search all of your local craigslists at once. Don't be afraid to drive a little ways for the right truck.
Pay more money for a solid truck. Don't settle for a rust bucket simply because it's cheap. It will cost more in the long run.
As far as desireable years, to each there own. If you ask 6 people, you may get 6 different answers.
GMC's have much lower production than Chevrolet, so you may have to look a little harder. The nice thing is that people seem drawn to Chevy's more, so your chance of finding a solid GMC is probably pretty good.