The latest aluminum tube construction with the interior profiles being shaped through hydro forming techniques and the resulting compliance has brought aluminum frames in to the comfort realm. Engineering in some resilience for vibration absorption makes them much less buzzy and overly stiff when so designed. The Synapse and Specialized aluminum Roubaix series are very compliant and their geometry is aimed at the endurance rider. They offer the advantages that a drop bar bike has with the slightly higher head tube and angles that give you a good, stable yet lower vibration prone than the racing oriented frames.
My CAAD 10 is not overly stiff and I don't hesitate to ride it over long distances. My set up is oriented towards the endurance with the bar height and stem length as well as using 700X25 width tires. A 700X28 in some brands is also possible. The Synapse, Roubaix, Domane and others will readily accept a 700X28 tire and even fenders if you want for wet winter riding. You can find CF frames at your price point but these are the lower end as far as their lay up in the finer matting and the resins used so they aren't as compliant or even as stiff in some cases, as the more expensive highly engineered CF frames. Steel and titanium are both viable options with several bikes in your price range, do not rule them out.
If you use BD to order a bike, or any of the on line dealers, be prepared to do some assembly, as said above, and possibly use an LBS to assemble and tune things. If you are skilled or are willing to follow a manual or video there is the possibility of doing all the set up your self. It is not difficult, even tensioning wheels can be dome with some patience and the right tools for the job. Some in bicycling do not like the idea of BD or the other on line services, that is your own personal call. I choose to use my LBS to buy and have the bike fitted properly even though I can work on the entire bike myself. the biggest draw back for on line buying is knowing how the particular model fits you and fitting it to your needs and build and fitness as well as its intended purpose.
Best of luck in making the decision, sorry for the length of my reply. Please keep us posted on the process and what you eventually decide to buy.
Bill
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Semper Fi, USMC, 1975-1977
I Can Do All Things Through Him, Who Gives Me Strength. Philippians 4:13