For serious hill climbing ability in most cases you really need to consider using a motor that drives through the bikes gears to allow gearing down down the motor for hill climbing (I'm assuming your folder has derailer gears on the rear hub). This is the cheapest decent quality kit I know of being offered:

http://www.sickbikeparts.com/catalog...roducts_id=170

I don't know if it would work on your folder or not (room available in the chain line where it needs to be mounted without interfering with the folding mechanism) but I thought I would at least provide the link for you. The kit is either a 650w "Cyclone" brand name external controller kit or a close clone of same.