Hey if you intend to upgrade all your components, it is very important to select a good frame. Also, do note the difference between some components that may not be easily upgradable (handlepost, folding latches). The Dahon will fold to a smaller package than the Stepdragon due to the shorter seat tube. The Dahon handlepost is much stiffer too.
For a relatively lightweight bike, the Vitesse D7 will be a good choice. Good platform to upgrade. However, note that the stock wheelset can only fit a 7 speed cassette, and the fork is chromoly not alu.
Actually if you can find the Vitesse P18 (~$1500), it is worth getting as the components are really decent, even if you do change them you can still sell them for a good price. Kinetix Comp wheels and alu fork. Got built in front derailleur mount also.
If you are willing to raise your budget much higher, the Vector X10 is one of the best you can get. Light, fast, looks great too! Just by comparing the components you can see le. But the price may just scare you off! ($2600)
Best way to decide is to go try the bikes! Or you can join us for one of our Sunday rides and try out all the different bikes at one go.
Regards,
Steve