What you will notice is the difference between a good carbon frame and an ally frame. I bought a Spesh Roubaix to replace my ally framed bike and the difference was amazing. No longer the numb hands through the road buzz, or the feeling of being shattered after a long ride. I rode LEJOG on the Roubaix last year and I'm glad I made the investment. Since then I have upgraded the components and the wheels to make it an even better bike. There are some good bikes on fleabay or gumtree to be had. I take your point about not trusting carbon frame from fleabay or gumtree, but if they are local to you go and have a look first. Domane is a good bike and come highly recommended.Would I notice much difference between the trek & carrera?
Good luck with your search and try not to get too confused with all this advice.