It is most definitely a 60-63. However in some areas if a 63 was sold in 64 it would be registered as a 64. No big deal in my mind, as long as you are aware of it and get the 60-63 versions of parts when needed.
Nope, not a panel. Panels have no side windows.
example.
as for being a GMC, definitely looks that way. Dual headlights and Dash are dead giveaways.
When looking at it, check for rust in the door pillers, around windshield and drip rails, rockers, floor, lower fenders, rear hood (near hinges), and rear quarters especially around wheel wells. These are the most common rust spots on these models.
Pimpston65 has the engines well stated. Based on what was said in the ad and pics shown. Unless something untoward shows up on inspection, definitely looks like a good deal. Good luck
BTW Looks like there are some handles bolted between the door and rear windows. Dont think its stock but looks like a good idea to me.