If you're not used to playing tunes fast enough (or slow enough) to dance by, then a metronome is indeed helpful to practice with. A good range is 112 to 120 bpm for contra. At the dance itself, look to the caller for instruction -- they'll tell you if you're too fast or too slow. Watch the dancers, too. If they're struggling with the dance, you might slow it down; if they're looking bored, speed it up. :)

You don't need a drummer; the music is rhythmic enough that even the melody players can express the tempo for the band to pick up. It does help for an inexperienced band to have a rhythm player, though -- guitar, keyboard, accordion, etc.

As a Scottish country dancer, I never had a problem predicting the tempo of the music from the chord. There's usually a couple of pick-up notes, or an in-breath that signals the beat between the chord and the tune.