I think a high-end, rigid, probably late 1990's MTB with slick tires and road gearing, possibly with a different handlebar setup, could be a good place to start.

I also like old sport-touring bikes for this purpose. But I rarely ever ride fast.

Mostly I would just avoid suspension, use a reasonable tire of whatever size you like, work out the appropriate gearing and an aero hand position, however you choose to do it.