The year will depend on the emissions testing in your state. Do they test all cars? Do they only test '96 and newer cars? Do they test at all? If they test for emissions, then that will limit what years to go with. Other than that, it's just a matter of personal opinion on which body style you like best.
The K-series is worth the most, and also cost quite a bit more than the other swaps. It's not a direct bolt-in swap for any car from '00 or older. So you'd have to buy a bunch of parts to make it work (mount kit, axles, custom header because it faces a different direction than the B's D's and H/F's; just to name a few parts). The B-series are the average swap, and there are more parts available for them than any other series engine. So really it depends a lot on how much you're really willing to spend in the long run. Though, this link doesn't cover the K-series swap, it will still give you a rough idea of what to look for:
http://www.automotiveforums.com/vbul...ad.php?t=19176