Dahon Boardwalk 6 conversion I wish to convert the 6sp to an 8sp.

Which model Dahon has the best 8 speed wheel unit to change the existing 6s speed wheel?

Also, what 3 speed hub gear would fit with the 6sp/wheel unit?

Cheers.
You don't need a Dahon specific wheel to upgrade from 6 speeds to 8. Bicycle wheels (like practically all bicycle components) are very standardized.

What you need is any wheel with:


The correct rim diameter
Acceptable rim width for the tires you want to use
Correct hub width/dropout spacing
8/9/10 speed freehub (slightly wider than those used for 7 speeds and below)


A difficulty I can foresee is that Dahon 6 speed bikes might use an older style 120 or 126mm wide hub rather than the now standard 130 or 135mm. An 8/9/10 speed hub that takes a cassette  will not be available in that width. Dropout spacing is very easy to measure with a ruler so you should check what you bike has.