windyrob is right
For the 100k or 200k rides then any bike is ok. It doesn't matter how comfortable it is, if it can carry lots of "stuff", how indestructible the wheels are, how wide the gear ratios are, if it has mudguards and tri bars or if it is made from steel, carbon, aluminium or Titianium.
Over 200k there is some agreement that the bike should be comfortable. This implies "wider" tyres, a good "bike fit" to your body, a relaxed geometry and a flexible rather than stiff frame.
Luggage carrying should be enough for a night time set of clothes, emergency food and a few tools/spares.
The wheels should be able to take a few pot hole hits and it's probably a good idea to have 36 spoke hand made wheels that will work fine with a couple of spokes missing. 20 spoke super light wheels are not good in this context.
Wide gear ratios are necessary. You need a big gear for speeding over those miles. Or you will never get there. Tiny gears are for the 2nd day of a 600 when you have 80km left, you can barely stay conscious, you are climbing a mile of 1:6 and your legs feel like old rope.
A good bike fit makes for more comfort than the frame material, how much padding is on the handle bars and a lot of other factors. And comfort is key
Mudguards are good as they keep the rain off. Fit a mudflap and you can be in a group following wheels in the rain.
Tribars give an extra hand position and are good for riding into the wind
Steel vs Al vs Ti vs Carbon for audax bikes? Usually the answer is steel or Ti. But carbon has it's fans too. And an Alu bike that fits properly beats a Ti bike that's just wrong