For me, everesting was also ‘about the bike’ as I love modifying stock bikes.
I chose the lightest frame I had – a carbon road frame.
Drive train / transmission: really, really, low gears: 11-46T, 11 speed cassette (Shimano XT).
This meant using an 11 speed mtb shifter (Shimano XT) and a rear mech (Shimano XTR) that would clear the 46T cog that I used once the legs turned to jelly (fairly soon in the procedings).
Power (the term is loosely used) was transmitted via an mtb crank (Shimano XT) with a 32T single chainring. As the crank was designed to fit mountain bike bottom brackets (and I was using it on a road bike), I had to use a headset spacer (Aerozine) as a shim to get it to work.
Other mods include: a flat bar (Ritchey WCS), flat bar brake levers (Shimano Tiagra) that were compatible with rim (gasp) brakes (Shimano Ultegra), ‘anatomic’ rubber grips (Bontrager) and (horror) bar ends as well as a 150mm stem (Ritchey WCS). The saddle (Fabric Scoop) was ‘the wrong size’ according to the measurements of my ischial tuberosities (@$$ bones), but it suited my bum (critical in a 24 hour plus ride).
Wheels: reasonably light clinchers (Shimano Ultegra) shod with 25C puncture-resistant tyres (Continental Gatorskins) and contrary to popular trends, the tyres were inflated to an eyeball-rattling 120 psi.
Pedals: ControlTech ‘flatforms’ - broad mtb pedals with plastic toeclips and leather straps (both Zefal).
Hydration: a single carbon bottle cage (Arundel Mandible) with various small(er) cycling bottles – a quarter full of ordinary water.
Second image is of the bike during the 8848. Also visible in the image are very tired legs and some junk food (banned by WADA).