Whether 26" or a 29er is faster is very much dependent on the type of terrain you're riding in.
Putting a bike together from a bare frame can be very rewarding - if the frame is sized right for you - but is usually also more expensive than buying a complete bike.

On bikepedia.com you can probably find the original spec of your bike.
Sheldonbrown.com and parktool.com has loads of repair and assembly instructions.