a: Yes, and no...upconversion is really a mis-term for a display that displays something at 1080p natively...totally different than what a true scaling chip will do to a signal.
b: If it has HDMI that accepts 1080p, you're set and "future proofed"
Generally, that number is ideal for screen to viewer...but take into account dimensions of teh room you'll have it in...a 50" deal will just look silly in a small room, even if ift's s full 12 feet away from the viewer.
Yes, they all still have an coaxial input for analog cable...though don't be surprised if the pic quality looks worse than a tube if you go that route.
Newer Plasmas do have reduced burn in but they are still over priced, use way more electricity, get much hotter, and still are recommended for "gaming".
It might be better to ask...what do you really want in an HDTV? A big screen? 1080p will do you no good if you hook up analog cable and a 360 to it.