I am probably not the best person to respond, given that I only bought my folder in February... but I got a good deal on a Dahon Speed P8 on craigslist. It was virtually brnad new
I have now put about 1200 miles on it, and have made some minor mods, but I love the bike. It is one of the 2 bikes I use for commuting (the other is a fixie). I have a long ride of 80 miles on it, and have hit the hills to make sure I can get up them on the bike. I purchased it with the idea of being able to pack it and take it with me when I travel-- without spending a small fortune on transporting the bike. SO far, I am very happy with it. I can get good speed on it, not as fast a s a full size, but not that bad...(no computer on it, so I really don't know how fast I was going). The Speed P8 has the advantage of a bigger chainring than the cheaper folders, so it has a better gear ratio span. I have yet to use the lowest 3 gears... I have cycled through them, to be sure the derailleur works, but have never used them on a ride.
Check with Thor, as he has a speed8 that says 'call for price'... so ti might fit your budget.
Can't say anthing about the others--
train safe-