The axle-to-crown measurement is the most important.
Your
current fork is probably 503 mm (the link is for the 2014 model, do your own measurement). That's the fully-extended measurement, it's a bit shorter when your weight is on the bike (suspension sag), so you can go with a slightly shorter rigid fork.
The Surly Disc Trucker fork comes in 376 mm and 390 mm sizes. That's a big difference from 503 mm. Your frame would be tilted forward several degrees with that fork, changing all the geometry angles.
The Surly Instigator (447 mm), Karate Monkey (468 mm), Ogre, (also 468 mm), ECR (also 468 mm), or Krampus (483 mm) forks would be a better fit for your project. They're all disc-compatible.
The Instigator is marketed as a replacement for 100 mm travel 26" forks.
But I'd get the Ogre or ECR fork; a bit taller and covered in attachment points for racks and cages.