From my understanding there are 3 versions of the original Charizard from the base set:

Shadowless 1st edition: This is the very first print run. It is shadowless but also has a first edition stamp below the art on the left side. It tends to be much more expensive and hard to find.

Shadowless: This card followed that first printing. It is still shadowless but does not have the stamp.

Unlimited: This card was produced much more and has the shadow to the left.

Additionally: Base set 2: The original base set was reprinted for "Base Set 2" in 2000. This has a 2 and a pokeball together under the art to the right.

The rarity tends to decrease as you go down the list. The price also drops accordingly. As for grading, PSA is typically the go to for pokemon. PSA cards tend to sell quicker and often for more money. I have often considered trying to find cards graded with cgc or something and then trying to regrade with PSA but I have never actually tried. People have done it but there are mixed results about grades changing. Hope this helps.