Put a least a 53 spocket on,the standard Downtube cranks beniefit from being upgraded anyway, so i would buy a complete chainset instead of a chain ring you tend to get a better deal. Dual drive hubs can be bought for about £25 on ebay if you need more gears.

The larger dahons with full sized wheels could be altered to fast road bikes, if you are lucky a Bike friday or Chameleon are findable for less than £500 also on ebay.

As regards folding forward reach bars, see "little pixels threads", or a quick release handle bar stem can be improvised. I use improvised low profile bars on my mezzo and Downtube bikes made by turning droped bars upsidedown and cutting them down.
See these bikes for examples on the "name your bike thread"
To fold my Downtube I lossen the stem and turn the bars so that the brakes go in to the spokes. On the mezzo it does not reguire this stage as I wanted to maintain the quickness of fold. I altered the folding angle on the mezzo, (think this can be done on a newer downtube?), and compromissed a little by used more cut down bars. I have not seen a folding bike with dropped type bars that is the same size as standard when folded, both my bikes are less compact now to fold, but I am happy with the slight trade off. I add some photos later.

There is a great looking DT mini on a preivous thread that looks like what you want to do, by "14R"

search for 16" wheel PERFORMANCE oriented purchase: DT Mini, Curve

justs my thoughts.