the Professional and Professional 2000 are two completely different models. The "Professional" is the older of the two and there's probably a couple of variations in existence. Although stamped "Professional", IMO this is a little exaggerated.
From what I hear, the Professional 2000 is the one to go for: it is the last model (together with the Leader and the Prestige) made by Grassi in Italy in the 80s/90s, before the factory closed. It has (on some models) the top F# key and balanced bell keys, unlike previous models. It was marketed as the "Intermediate" model, however a lot of people prefer the sound to the "Prestige", which in my opinion is more suited to classical.
Also don't be confused by the "Model 2000", again a different model than the Professional 2000.
There you go: more than you ever wanted to know about the Professional 2000! If you want to look up serial numbers t ofind out when it was produced, do a search on SOTW: a kind Italian gentleman, posted a list there.