Thinking about a CX bike
So I've been looking for a bike for a few weeks now (though with no car and no bike it feels like it's been MONTHS) and finally I found what I consider to be a good deal on CL.
I found a 2008 Motobecane Fantom Cross for a little over 400 with an upgraded rear derailleur to 105 (from tiagra I assume)
Now, this seems like a good deal to me and I can bypass all the misgivings about buying from BD by riding the bike in person. (aside from the fact that I dont REALLY know what to look for in a used bike, I have some ideas).
First off, I'm probably between 5'10'' and 5'11'' and I'm worried if it will fit me. I'm a LITTLE worried about the size because the guy posted that it's a 57cm (though I thought they only made them in 56 or 58) and I think a 56 would fit because my reach seems to be most comfortable on a bike with a top tube around 56 to 57 CM and looking at it that is exactly the 56 and 58 top tube length. I just don't know that much about bike geometry so I was kind of wondering if the fantom frames ran big or small or compact or whatever.
The other thing I'm concerned about is weight and tire / size. How much did the '08 fantom cross weigh? and how much does that really matter to me as a beginner. And will I have the option of putting on say, 700x28 road tires on it for slightly more speed?
 This isn't a CX oriented question but one of my worries about buying used is that it might be stolen, so does anyone have any ideas for good ways to detect the integrity of a bike, (other than look for the Serial number that's usually stamped on the crankcase?) and the structural integrity of the bike?
The bike will be used as a commuter to school and for recreational riding. I was initially looking for CX bikes but I gave up due to their price being so high normally but if I can be the first to check out this used one I may just have to pick it up. (as long as it feels good riding it, doesn't looked like it's been wrecked and doesn't seem to be stolen.).