I ride a Speed D7 for upwards of 50 miles on a day's outing. First, I switched to a 2.00" Big Apple upfront (it'll fit under a fender but just) with a 1.75" Marathon in back for added protection. I've tried a number of Brooks saddles as well as the Selle Anatomica and find them stiff/uncomfortable even after a thousand or more kms of break-in. After reading that Tour Divide racer Marshall Bird used a cheapo Forte Softail from Performance Bikes on the 2700+ mile course a couple of years back
Forté Men's Softail Saddle , I tried that on my Dahon. Much better, especially at low temps because it has firm padding which provides some insulation (good to -10C). If you have any problems with it or damage it, they're inexpensive to replace.
I don't need a Thudbuster and doubt you do. I've replaced the plastic pedals with FD-7s and installed Ergon handlebar grips. You can also switch out the bottom bracket cheaply using a Shimano sealed BB or equivalent:
Upgrades on the Dahon Speed D7 .
Done!