Replacing suspension with a solid fork
It sounds like you've sorted out the first issue, headset size.  You'll need something with a  1 1/8" (also seen as 1.125" or 9/8") steerer.

Your bicycle's handling is based on the geometry. Changing the axle-to-crown distance will change the angle of your head tube, altering your bicycle's handling significantly.  The other major characteristic of a fork is "rake", or the distance that the axle is in front of an imaginary line down the center of the steerer.  Within 5-10mm of rake change you should notice a fairly minimal effect on the bike's handling, which is good because suspension fork rake is rarely published and measuring it directly is difficult.

The axle-to-crown distance is one factor, to get that right - assuming your bike has an equivalent fork - you should be looking for a "100mm suspension-corrected" rigid fork.  In practice this means a fork with an axle-crown distance of about 455mm.

If you want a 100mm suspension corrected, 26" wheeled fork with disc brakes that also has rack (I'm assuming you mean midfork low-rider rack mounts) and fender mounts, your options list is relatively limited.  The new Surly Troll Fork is likely to be one of your few options.