How much are you willing to spend on mods? How light do you want to go?
A $600 folder with $600 of mods will likely still be heavier than a $900 folder with no mods.
Many folders also use proprietary parts that cannot easily (if at all) be swapped for lighter/better alternatives. I know you don't want 16" wheels, but as an example A top end $2000+ part titanium Brompton with all the lightweight extras (Brompton specific) is still no lighter than the Mezzo D10 off the peg at half the price, but you don't easily have other options for upgrades.
Easy and cheap ways to save weight on the bike you link to though are saddle, bars, pedals, or even remove the gears, but it will still never be light. Unfortunately they even use their own folder specific rear mech which is generally poor and cannot be swapped for anything better if keeping the gears. Dahon (in the UK at least) have had a poor track record for quality recently and very poor after sales care.
Maybe still above your budget, but http://www.kansi.co.uk/bikes/
might be worth a look. Otherwise, I would seriously suggest you buy good second hand to keep within your budget.