Have a look at the bikes on www.downtube.com They would be affordable for your customers and great on climbs and for the kind of mileage you are talking about. The Birdy and Airnmal brands are very good but very expensive options unless money is no object or they will be using these folders all the time after their vacation. The company owner of Downtube Yan Lyansky also provides excellent support and advice. You can see one review of the full suspension folder on the UK site:
http://www.foldsoc.co.uk/Mike/downtube.html
I have the hub geared and full suspension bikes. Both are excellent in my opinion. The 8 or 9 speed gears offered have a sufficient range of gears for touring. Most people outside of professionals or very serious racing/offroad enthusiasts use every gear on 16/21/24 speed bikes etc. Maintenace is easy at they are 20" wheel folders and use standard components widely available at many specialist bike shops across Europe. These bikes can be easily packed in a suitcase, especially with front wheel taken off and are easy to fold into cars very quickly. They are a similar ride to the world's largest folding brand Dahon but half the price. I believe the Downtube VIII H 2008 would be the best if they need to use panniers as it has a rack but the FS is great for trail riding if they are happy to use a good rucksack/bacpack when combining a car and day cycling before returning to where they are staying.