Seems a good idea if you cant get a dualdrive hub cheap. However you don't need a new chainwheel or crank, don't understand that?

The disadvantage of the Dahon idea is that you will not have a really compact fast folding bike that's upgraded, but you have the advantage of a second bike. I prefer to have one flexible package, although I have 4 folders, I mainly use the black Mezzo.

It need not be as expensive as you think.
The 3X7 wheel cost £37 gross as I bought a mission space genie bike with a 3X7 fitted and swoped it for a mezzo rear wheel then sold that bike for £37 less.
The Gold 3x7 mezzo cost £389 brand new. Therefore the total cost to buy and build this dual drive mezzo = £426 including bike.

You just need to look on ebay and get a wheel built up with a bargain hub.

my other experience
Used mezzo bought with intention of upgrade £379 on ebay imaculate bike with loads of extras (quality d locks,lights,waterproofs,tools,speedo,

£100 dual drive wheel for MBT unused

£100 wheel build and lovely sram 9 speed close ratio block
rim£30
bars £ 5 used

triagia str brakes shifters 3X9 £ 60 used immaculate

cables,bits and bobs £30

total £709 including bike with bag and loads of spares.

however ,sold rear wheel for £25,sold MBT cluster for £25, total = £659


I know what you mean,upgrading can be a headache, but I am extremely pleased with my high performance compact folder! There are a issues with going for the dual drive, but it ends up being a unique bike that folds very compact ,very quickly, and has much of the dynamic capability of larger bike. I will post some recent pictures as I have a camera arriving in the post soon!

Their is a tread I started on Mezzo upgrades if anyone wants to post any upgrade ideas there also

http://www.bikeforums.net/showthread.php?t=510351