Currently there are no front suspension forks I would recommend. Only the GA force kilo is good, but cost >$600. Better go for Big Apple tires and refuse a shock. Simpler. Lighter. Better.
If you absolutely need suspension, the Dahon Jetstream XP is a good option (features the GA force kilo fork). Also the rear suspension works almost impact-free.
07 Schwinn Voyageur GSD, Next Avalon, 2007 Dahon Yeah
I have the Downtube FS8 model and I really dig the ride. It is very cushy all around. It comes equipped with smooth tires so the speed penalty is not as bad as you would think. The rear suspension is adjustable so you can tighten it if you want less cushiness. The front suspension is not adjustable, though.
I second the notion of Big Apple tires in 2.0" or better 2.35" (may be too wide for some bikes). Pump them to tire pressure you like and ride them. Lighter, easier to maintain, simpler than a bike with suspension.
These tires are made with kevlar for puncture resistance. Also known as balloon tire technology, they are designed to be filled with a lot of air to provide shock absorption while producing very low rolling resistance for the width of its tires. The Big Apples have lower rolling resistance than many tires that are in fact, narrower in width - counter-intuitive.
These newer style balloon tires are now being used in a wide variety of bikes ranging from cargo bikes that carry heavy loads to cruiser bikes. Many people are surprised at how easy, how fast and how smooth the Big Apples run.
Do a google search for big apples and see what people are saying about them.
Here's to get you started:
If you go for the Downtube, I believe they can sell you a non-suspension fork for the bike (they do have one listed on their website). You might try talking to them to see what options are available to you.
I recommend the Downtube. If you enjoy riding the bike, your going to ride for longer than you may expect at the moment. Comfort will become a larger factor the longer you ride.
I have a Dahon D3. I ride 3 miles in the morning and 9 miles in the evening (Up from 3 miles in the evening) as I've come to enjoy traveling to and from work this way the most. I have Big Apple Tires, Brooks sprung saddle, Ergon Grips and heavily padded gloves to improve comfort.
I still suffer with pain in my wrists and neck caused by vibrations (although much improved over the basic spec) even with these upgrades. (Big Apple tires come with the bike).
If I was you, I would buy a 20" Downtube with shocks.