Titanium, or steel for comfort. Something you can ride in comfort for long hours at low speed. Most of the 5000m+ passes I did, took upto ten hours to ride up at 3mph in the smallest gear on my mtb because of the altitude.
Tyres, something with a good volume which rolls OK at lower pressures to give some comfort over the ripples. They are a real killer and sap any momentum, especially with a pounding headache from altitude sickness. Note its not all gravelly tracks – a lot of the time these are covered in upto 6 inches of dust which is worse than riding in sand. When a truck passes you can get lost in the dust cloud for minutes (always keep a buff round your neck to pull over mouth & nose for this). I fell off the edge of the road once in a truck dust storm.
I did it on a titanium inbred.