Sadly, just because it's alluminum does not make it lighter or better, it does slightly increase the likelihood of a stress failure if said fork isn't heavy enough to resist most flex.
Sure rim wreckers work fine on a small wheeled bike, yet they are destroying the wheel at 3 times the rate of a big wheeled bike. A 140mm disc is pretty small compared to a 349mm rim, the weight difference is negligible when you factor in heavier rims for rim brakes and the fact that it moves the weight toward center of the wheel. Mechanical discs are much easier to maintain, repair and rebuild than rim brake setups too.
Never had headset problems with the Brompton. Then again , teflon bushing can make an excellent headset done right , it doesn't go around much.
They already dealt with the rust out problems, probably 20 years ago.