New front derailleur I need to replace the front derailleur on my friends bike, this one is old and rusted, its a shimano FD M191. I've read a bit about what I need to know before I buy a new one but I just wanted to be sure before I make the plunge. Please excuse my newbiness...

The current one has a down pulling cable fix and serves 3 cogs, from what I gather these are the only factors that I have to ensure my new derailleur caters for. Is this correct? Can anyone recommend me something sub $30 that would work, and is there anything else I need to consider?

Thanks.
There are several criteria when selecting a front derailleur. Top, bottom and now side cable pull. Some Shimano models are both top and bottom compatible. The mount type, seat post clamp mount (correct diameter or have an adapter), bottom bracket mount or direct mount. The derailleur must be compatible with your crankset size. This is usually specified by a maximum number of teeth. It must also match the number of ring gears on the crank (double or triple). It must also match the chain 8,9,10 etc, speed. There is some overlap on 7 and 8 speeds that will work with both.