The risk of a question like this is that people tell you why their folding bike is best. Most of us can only really confirm that we have the best solution for our own use. there are pros and cons to all the different wheel sizes, gear systems, frame styles, manufacturers etc. The best advice I can think of is already given above. Don't buy cheaper that Dahon/Tern. These are worth every penny and won't disappoint. And a good folder keeps its value if you find you made the wrong choice. A cheapy is worth nothing.
Hope this helps you a bit.