Should I buy this one? Price steep? 2 days to make a decision.

Original, with correctly signed crown and what appears to be a correct crystal. Dome shape and logo seems a little funky so it might be a modern replacement, but it seems alright. Heads-up that this is a later model, so the indexes are painted black instead of being inset with slices of Onyx stone. It also has a 564 movement, which IMO has less attractive finishing than the 561, but you do get a pull-type quickest whereas there is none on the 561.

Your movement and case seem to be in very good condition, with only a few dings and scratches and most of the original contours intact. As you can see, the medallion still has its matte finish in the lower portions – This is typically the first thing to go when the case goes under a polishing wheel.

Given that it comes with an original buckle and bracelet, IMO it's a fair deal, especially with how Constellations in good condition have been gradually moving up as of late. That said, maybe see if you can negotiate the price down $200-300 due to the later make. It never hurts to get a better price.

If you're on the hunt for a Constellation, definitely do some research here: http://omega-constellation-collectors.blogspot.com/
Those few minutes you spend reading those articles may save you hundreds of dollars.