Any decent headset should work for touring. Pay extra attention to it if you spend lots of time off-road.
All steel forks weigh ~1300-1800g when new, and a little less if/after you hack steerer tube. Have a friend help you measure ATC distance (you sitting on bike, sus fork slightly compressed), then find a rigid steel fork with ATC preferably within 10mm of the original dimension. Rake is usually 45-55mm on most all of them.
You need steel for touring if you intend to ride off-road and/or carry a front load. Do yourself a favor, just buy the Surly fork if the ATC is close - you won't find anything lighter with adequate strength/toughness/longevity for the relatively low cost of a Surly fork.
Adding rim brakes to the Dahon would probably cost half as much as a whole new LHT frameset, or more. This estimate excludes the cost of a decent respray/re-powder-coat to repair damage to finish from paint stripping/welding/brazing.