Dont get used! especially used carbon! search for waterrockets "considering buying a used trek" thread. buying used means you have 0 warranty. even if its a glaring problem thats obviously a quality control issue. No OEMs are going to warranty ANYTHING! just want you to be clear on that. you will be on your own. Have i hammered that one across yet? oh, just so you know, they WONT!!!! help you if your bike breaks and it's their fault. got it? good!

Buying a new bike, you have an insurance policy, and typically the lbs will be more helpful, and not nickel and dime you for that quick wheel true, or simple adjustment. plus, i bought a brand new road bike for $200 cheaper than your high end budget, and while it mainly has sora components, its not too bad. right now i need the break in tune up, and the only problem i'm having is slow shifts from cable length(derailers scare me still, i wont adjust them myself yet), and it seems to occasionally drop the chain when i back pedal to get my foot in(only happened 2x in a month tho). I really suggest new, with how good the components are, even the stuff most people scoff at here, really arent bad. theyre just not the epitome of perfection. Also, buying new, you have the size you need, the bike you really want, and my lbs fitted my bike to me(not sure if all do).

I cant really help with detecting if "x" component needs replaced or not, so ill leave that to the guys who have biked farther than ive driven in my life.

I know i didnt really answer what you asked, but it seems to be something that slips peoples minds. hope it helps