The Decathlon looks very short in the 'top tube'. Have you had a look at C H White's shop? I realise you are in France, but I would think they could ship to you.They are folder specialists and also stock spares, so if you bought a used one, you could potentially upgrade/repair as required - there are a lot of used Dahon Speed D7s (steel frames) around, which could be used to build up as a tourer.
http://www.foldingbikes.biz/I managed to get a little used, immaculate Dahon Speed TR 24 speed, on Gumtree, for around the £400 mark and I love it. I have used it for day rides, train journeys and local loaded trips. It's extremely comfortable and really does ride like a real bike..it also has an adjustable 'stem' and ergon bar end grips, so there is a degree of adjustment in reach. Mine came with a klikfix fitting and bar bag, together with the proper full size rear rack. The Joules dyno hub works flawlessly, although I have changed the headlight to B&M cyo LED as the supplied halogen light was inadequate.
I haven't toured on it yet but plan to do so. Here's mine, complete with tent on rear rack for illustration purposes!
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IMG_5552 by
hop58, on Flickr*]
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IMG_5554 by
hop58, on Flickr*]
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IMG_5555 by
hop58, on Flickr*]