Xhim, folding bikes can go quite fast if you can gear them properly -- but that can be challenging because the smaller wheels present some more difficult gearing scenarios.

For under $800, I would suggest a Dahon Speed P8, sold by Thor. I would get the Tangerine model, which has nicer components; though I'd personally swap out the tires for Marathon Big Apples or maybe some other medium-fat tire. I would also consider asking Thor about the cost to mount a decent rear rack so you could attach panniers for biking to work.

It is possible that there is an equivalent Tern in this price range, but I don't know what it would be. Thor would know however and could tell you directly, call him at 1-800-222-8356.

This bike's top gear will be more than plenty for a typical commuter, but not for a racer.

I would not get a Citizen nor a Crane: I do not think they are of sufficiently high quality.

If you had more money to splurge, I would look at a Dahon Mu P24, which has better gearing.

As to chainguards -- like most street bikes, folders don't have plastic straight chainguards. Instead, you should just get a strap to keep your pants close to your leg. Any bike store sells them.