If I were building a carbon fiber bike to do any degree of hammering, I wouldn't go as old as 2008. The technology continues to advance and a 2008 model is just obsolete.

If you're determined to get it, you'll want to check for signs of fatigue or delamination. My Tarmac is a 2011 and it is starting to show its age. Years of hard riding and flexing where the aluminum dropouts and bottom bracket join the carbon have caused all the paint to come off in that area. I check it regularly to make sure the joint isn't actually cracked. So far so good.

As far as the Elite model goes, I think think the difference over the Comp was in the components. I don't think the frame is upgraded until the s-works level. Could be wrong on this.