The difference between the Downtube and normal Helios is negligible. However the Dahon Helios SL is a sub 8kg bike and has very high-end components. If you can get hold of one of these it is a different proposition and well worth looking at.
I recall that I originally paid about $760 (Canadian) for it... I think before tax.
This was in May-June 2006, from my LBS.
Great bike - I've gone 100-120km on it several times now, although 60-80km is a distance that I do more regularly. Obviously with the tall handlepost you are not able to really throw your upper body into it a great deal like a Tour de France rider... I'm not sure this is really all that different on other Dahons or similarly designed folders. But I'm not particularly gentle or slow with it and my bike has served me well...
I would draw the line at a 2005 Helios P8... earlier models used the older style handlepost etc, which (to me anyway) make 2004 models less desirable.
I've really run the gamut of upgrades on this model - it was great as a stock bike and every little tweak or upgrade just made it that much better.
Good luck - hope you can get a good price on it.