Fenders have nothing to do with the fold per say, but they stabilize the bike in the folding position. The small wheel on the rear fender combined with the easy wheels create a tripod affect to keep the bike upright when folded. This small fender wheel also allows the bike to roll along the ground for short distances.

As for gearing it certainly is a personal choice on what is appropriate. When I was contemplating a single speed, the sales person suggested that since you are going to have the chain tensioner anyways, you might as well get a two speed. It doesn't add much weight and still looks pretty good. In the end I purchased the 3 speed and now with hindsight, I should have went with the two speed since I only use two of my three gears.

I did test ride a single speed, fenderless Brompton, with the fast tires and there was a noticeable difference between this and what I bought. The single speed seemed so much lighter and faster.

Again it all depends on what you are going to use the bike for.