Those are good questions. For the first one, you might want to search through the "For Sale" forums and see what prices people have posted for Silvios. Not to say that is what you will pay, but it will give you an idea of the historic pricing. I know you are probably looking for a solid number, but I don't have one. Maybe another member can chime on that.
Ultimately i would look at my budget in relation to what it will buy today. If you were to look at what a new S40 goes for, then your budget is some percentage of that. Cross referencing what a new S40 costs with historic Silvio sales listings, you should get an idea of what used bikes cost in relation to the a new one.
As for how to tell the different generation of Silvios apart, a web search will get you photos, videos and reviews of each example. You'll also find assembly manuals, which I have found very useful in telling the difference between versions of Cruzbikes. As an example I've attached a Silvio 1.5 manual to this post. You'll see one of the distinctive features of the Silvio 1.5 and the 1.0 are the carbon leaf spring stays and dual suspension. Hope this helps.