They're worn very often: at least once a term for subject dinners, balls, sports team/music society dinners etc. You'll probably wear it more if you're in lots of societies or play lots of sports.
So yeah, buy a tux. You can find jacket and trousers for about £150, and dress shirt/bow tie will be about £35. It might seem a lot, but either you go to black tie events in a lounge suit, or hire every time - the former is uncomfortable and the latter is expensive!
You're right, the key features are the satin lapels and stripes down the sides of the trousers, but no one will be checking to make sure you have bought the standard formal wear. Normal is three buttons on the jacket, but just go with what looks good! You get points for wearing something slightly different at black tie events.
Ask in a formal wear shop if you're unsure about any of these things. And consider some kind of waistcoat - not many people wear them, but if you can pull them off, they're pimped.