I use a rack and single pannier on my Bike Friday. It works perfectly, does not affect handling (can feel it slightly but very soon get used to it) and does not greatly affect the fold. The BF puts the rack way high off the rear tire, and due to the specific way the BF folds (rear triangle/tire fold under the main tube) the rack does cause the rear tire to sit higher off the ground when folded. Your Dahon will be fine. I also have a Speed P8 with a rack, and it does not affect the ride or fold. Get a pannier with a shoulder strap if you have distance to carry it. You'll be just fine with a rack and pannier.