I'm not sure whether many of the available electric folders can really produce enough power/have a big enough battery to be really useful. I think you will find the zap very dissapointing. I think the quoted weight and maximum speed optimistic to say the least. For hill climbing you really need 36V. However I believe the curry electric conversion kit is a cheap, usable option.
Heinzmann produce a kit which is very powerful and has about a 50 mile range.....at a cost. But I have seen some very neat conversions. Kinetics of Glasgow supply and fit the kit. http://www.kinetics-online.co.uk/
I've attached a picture of a dahon conversion. Incidentally this bike is for sale on the forum.