Fit and comfort should major priorities, for what to use; I’ve only been Audaxing this year, but have seen everything from good old 531 toruers, Alu, Ti, Bents, Trikes, Tandems and Moultons, its what you are comfortable with.
the most recent 600 I did there were guys on carbon road bikes who did it fine.
As long as it has braze-on’s for racks, mudguards, decent lights, and gearing, the frame material isn’t too important.
Currently I use a Planet X Ti Sportive, not perfect, but works, and occasionally a Salsa Vaya, which is a lot heavier, but with the right gearing keeps up with everyone.
Have you looked into the Kaffenback Mk2 (not out yet), this looks interesting with discs, and should be cheaper that the Vaya.