You need two rear 26" 17.5mm offset wheels. The stock fork is spaced for a 135mm hub; this allows you to have a backup wheel should you find yourself in the middle of the Sahara desert with a broken cassette. Many folks choose a single speed or fixed disc hub for this application.
You will need an E-type Front Derailleur. This style of Derailleur mounts to the bottom bracket negating tire clearance issues associated with traditional ders.
You will need to use two rear brake caliper adaptors (for stock frames with the curvy legged, 135mm fork).
You need two rear brake calipers for your two rear wheels (some calipers are interchangeable front and rear and some are not).
You will need a 100 bottom bracket and corresponding crankset. Because of the need for massive tire clearance at the chain stay junction we use a 100mm BB shell