The OP said he wants to go as fast as possible "but be reasonably comfortable".
This can easily be achieved by using supple tires with latex tubes or a tubeless setup. Having a 25mm tire with a wider internal width rim or a 28mm tire can be very comfortable.
If it were me. I would probably buy a new bike with $3500. Either find a used Tarmac or TCR or build my own. There are a lot of good quality open mold frames and wheelsets out there to choose from